I have noticed that terminology sometimes varies slightly across domains of study, depending on whether I read from a linguist, lexicographer, or educator. School curricula also differs, depending on the publisher and the grade level. This can be disconcerting. It helps to stay flexible.

Below, I define the terms that occur frequently in this website Where quotes are provided, I refer primarily to two sources, What is Morphology (Aronoff & Fudeman, 2005) and A Glossary of Morphology (Bauer, 2004). 

affix: “A bound morpheme that attaches to a root or a stem to form a new lexeme (derived form) or an inflected form or stem of an existing lexeme (Aronoff & Fudeman, 2005, pp234). “There are various kinds of affix, named according to their position in relation to the base to which they attach. The most common are prefix, attached before the base, suffix, attached after the base, and infix, attached, inside a base” (Bauer, 2004, pp. 13–14). 

compound: “A derived form resulting from the combination of two or more lexemes, e.g., space + ship > spaceship. (Aronoff & Fudeman, 2005, pp 236). “A compound is a lexeme which contains two (or more) stems and which does not have any derivational affix which applies to the combination of stems. Thus [grand-father] is a compound while [[grand-father] ly] is a derivative whose base is a compound. The criteria for distinguising between a syntactice construction and a compound are not well established, and sometimes lead to dispute as to what is or is not a compound” (Bauer, 2004, p. 32). 

concatenative: “A term that describes morphology that builds words by the linear addition of morphemes” (Aronoff & Fudeman, 2005, pp236). For example, the word replacements is a concatenation of re- + place + -ment + -s. 

derivation or derivational morphology: “That type of morphology which creates new lexemes rather than forms of a single word (compare inflection). Derivational morphology is the least syntactic type of morphology: its role is not to mark words as having particular grammatical features, but to create words from a known base which can fit into a different syntactic context (Bauer, 2004, pp. 38–39). “The creation of a new lexeme from one or more other lexemes through the application of some morphological process, such as affixation or compounding. Also called lexeme formation and word formation. Derivation contrasts with inflection” (Aronoff & Fudeman, 2005, pp 237). 

derivational suffix: When a derivational suffix is added to a form, a new word is created. A grammatical change may occur, too, but this is not a necessary characteristic. An example of a derivational suffix is -ly, which tends to create an adverb, as in slowly and carefully, and -ous, which tends to create an adjective, as in joyous and spacious. derivative: “A derivative is a word coined by a process of derivation” (Bauer, 2004, p. 39). 

functional shift: A functional shift is the process by which an existing word or form comes to be used with another grammatical function (often a different part of speech); an example of a functional shift would be the development of the verb scrimmage from the noun scrimmage. A functional shift takes place when a word goes through a process of zero-derivaion/conversion. A functional shift may also occur when a derivational suffix is added to a base. For example, when the suffix -ness is fixed to the adjective soft, a functional shift takes place, and the word becomes a noun, softness.

inflection: “The formation of grammatical forms of a single lexeme. Is, are, and being are examples of inflected forms of the lexeme BE” (Aronoff & Fudeman, 2005, pp 238). 

inflection, or inflectional morphology: “The morphology which provides different forms of the same word to show the role that word plays in a sentence … Also, the affixes which are added to stems for inflectional morphology are also called inflections” (Bauer, 2004, p. 55). 

inflectional suffix: The suffixes which are added to stems for inflectional morphology, typically including the suffixes -s, -es, -ed, -ing, -er comparative, -est superlative (but this is not exhaustive). Contrast with derivational suffix.

lexeme: A unit of lexical meaning, which exists regardless of any inflectional endings it may have or the number of words it may contain. Thus, fibrillate, rain cats and dogs, and come in, are all lexemes, as are elephant, jog, cholesterol, happiness, put up with, face the music, and hundreds of thousands of other meaningful items in English. The headwords in a dictionary are all lexemes” (Crystal, 1995, p. 118). 

morphemes: The building blocks of words, indivisible elements of meaning such as affixes and roots. 

morphological awareness: Morphological awareness (MA) has been defined as “awareness of and access to the meaning and structure of morphemes in relation to words” (McBride-Chang et al., 2005, p. 417) or, “The ability to reflect upon and manipulate morphemes and employ word formation rules in one’s language” (Kuo & Anderson, 2006, p. 161). Morphological awareness includes both knowledge and awareness, increases over time through exposure to varied word types, and is a special type of linguistic insight. 

morphology: One of the universal components of language, found to some degree in every language. “Morphology is the branch of linguistics that studies word formation, including the mental system involved with understanding how two or more symbolic units can join to create a word. Morphology deals with words, their internal structure, and how they are formed (Aronoff & Fudeman, 2005, pp. 2–3, 239). 

prefix: see affix. suffix: see affix, also see inflectional suffix and derivational suffix. 

zero-derivation (conversion or morphological conversion): “A conversion [or zero-derivation] is the presumed derivational process which takes place when a word which normally occurs in one word-class takes on the characteristics of a different word-class without any change of form. Thus, in isolation we would probably say that empty in English is an adjective, but in Empty the bottle! it is a verb, and in Kim removed the empties it is a noun. The latter two examples illustrate instances of conversion” (Bauer, 2004, pp. 36–37).


Some additional unreferenced ramblings off the top of my head...

Morphemes: Bound or Free    Morphemes are indivisible units of meaning, such as prefixes, suffixes and roots. They are classified as bound or free.  A bound morpheme does not stand alone as an English word, unlike a free morpheme. For example, in feudalism, the suffix -ism is a bound morpheme, but feud is a free morpheme, because it can stand alone as a word. A compound word like lighthouse has two free morphemes--light and house--two base words. Read here for a lengthier discussion of compound words

AFFIXES:  As a noun, an affix is typically either a prefix or a suffix, at least in the English language. As a verb, it means to attach or place in position. Affixes are bound morphemes, but some have also become words in their own right. For example, the prefixes sub- and dis- have become informal words, slang, commonly used.  So did we free those morphemes?
Pronunciation: To pronounce the noun form of affix, we put the stress on the prefix, saying /af--fix/ with a "short a" sound.  However, for the verbal meaning 'to attach, to fix in position' we put the stress on the root /uh--fix/. When we build words with prefixes and suffixes, we affix the affixes to a base.

Prefixes are affixes. A prefix is located before the root or base and a suffix is located after the root or base. Thus, in reteaching, re- is a prefix, TEACH is the base, and -ing is a suffix. In prenatal, pre- is a prefix, NAT is a root, and -al is a suffix. Some words contain multiple affixes surrounding a root, as in unpreDICTability and paraproFESSional.

Suffixes are also affixes. Suffixes come in two categories, either derivational or inflectional. By adding a derivational suffix we create a different word, but inflectional suffixes only create a new form of the same basic word. If the base word is girl, the inflection is girls with the addition of the inflectional suffix -s. The derivation is girlhood, with the derivational suffix -hood. Girlish, girlishly, and girly are also derivations -- new words formed when derivational suffixes are added, which often changes the word class (grammatical function, part of speech) as well as the meaning.
  • Inflectional suffixes do not change the grammatical category or meaning of the word to which they are added (car is a noun; we add -s, making cars, we still have a noun, still a vehicle). Inflectional suffixes are learned in primary grades and include -s, -es, -ed, -ing, -er/-est. A few suffixes can serve as either derivational or inflectional, as with the suffix -er
  • Derivational suffixes are more advanced because they shape meaning and nuance-- creating a different word -- and they have the potential to change the grammatical category of the stem word. For example, the word sunny has a meaning related to the basic concept of sun but the derivation sunny is not a noun, despite the fact that it is contains the noun sunSunny is an adjective, because the derivational suffix -y often produces adjectives, as in happy, persnickety and zesty (also in jesty, were it a word). There are dozens of different derivational suffixes, such as -y and -ly; they each play a role in directing word class (syntax, part of speech). We do not need to learn them all -- only the most productive ones. 

The core element of meaning in a word is called the root. A root is typically, not always, a bound morpheme; in order to become a word it must be bound to an affix. Example: The Latin root ject means 'to throw' but it is bound; it does not become an English word until we add an affix. Through affixation, we derive projectile, ejector, dejection, reject, etc.  Roots can be traced back, etymologically, to source languages. Most English words stem from Latin (the language of the Roman empire), French (which is based in Latin because the Romans colonized that area and brought their language with them, as with much of Europe), German (which is related to Norse, Danish, etc.) or Greek (from whose language the Romans freely borrowed). English also contains words from dozens (probably >100) of other languages, but most English words originated from Latin, French, Germanic, or Greek roots. The English language traces back to the Mother Tongue, the somewhat hypothetical Indo-European Roots.

A base word is a word (thus a free morpheme). We can add a morpheme to a base word; for example, the base word in starry is star. The terms base word and root word are often used interchangeably in educational literature; in each case, they signify a simple, or basic, English word having only one morpheme, such as farm, yellow, sign, pocket. In some reading programs, children might be asked to, "Find the root word in farmer" (farm). In other programs, they are asked to, "Find the base word in signature" (sign). Note: Some Latin roots have become basic English words (act and port for example).

A base (not to be confused with base word) is a neutral term used by linguists, designating some core to which morphemes are added to construct a word--so a root like ject could serve as a base, but so could a base word like star.

Morphologically simple or basic words must be swallowed whole because they have only one morpheme (green, shoe, umbrella, mistletoe) but morphologically complex words contain more than one morpheme and can be parsed, deconstructed, decomposed. Complex words are internally coded with information about the word's basic meaning and the word's syntax.

Learn more about derivational suffixes

Explore a plethora of terms pertaining to linguistics, including morphology, phonology, grammar, syntax, semantics, etc.

Click this link to read from the new Popular Linguistics Online, where Corrine McCarthy (George Mason University) describes how words are formed and provides a definition for many morphology terms. 

Also see linguistic terms here