A morpheme is the smallest grammatical unit in a language. A morpheme is not identical to a word, and the principal difference between the two is that a morpheme may or may not stand alone, whereas a word, by definition, is freestanding. The linguistics field of study dedicated to morphemes is called morphology. When a morpheme stands by itself, it is considered as a root because it has a meaning of its own (e.g. the morpheme cat) and when it depends on another morpheme to express an idea, it is an affix because it has a grammatical function (e.g. the –s in cats to indicate that it is plural). Every word comprises one or more morphemes.
Every morpheme can be classified as either free or bound. These categories are mutually exclusive, and as such, a given morpheme will belong to exactly one of them.
Bound morphemes can be further classified as derivational or inflectional.
Allomorphs are variants of a morpheme that differ in pronunciation but are semantically identical. For example, in English, the plural marker -(e)s of regular nouns can be pronounced /-z/ (bats), /-s/, (bugs), or /-ɪz, -əz/, (buses), depending on the final sound of the noun's plural form.
Generally, these types of morphemes have no visible changes. For instance, the singular form of sheep is "sheep" and its plural is also "sheep". The intended meaning is thus derived from the co-occurring determiner (e.g. in this case "some-" or "a-").
Content morphemes express a concrete meaning or content, while function morphemes have more of a grammatical role. For example, the morphemes fast and sad can be considered content morphemes. On the other hand, the suffix –ed belongs to the function morphemes given that it has the grammatical function of indicating past tense. Although these categories seem very clear and intuitive, the idea behind it can be harder to grasp given that they overlap with each other. Examples of an ambiguous situation are the preposition over and the determiner your, which seem to have a concrete meaning, but are considered function morphemes because their role is to connect ideas grammatically. A general rule to follow to determine the category of a morpheme is:
Roots are composed of only one morpheme, while stems can be composed of more than one morpheme. Any additional affixes are considered morphemes. An example of this is the word quirkiness. The root is quirk, but the stem is quirky which has two morphemes. Moreover, there exist pairs of affixes that have the same phonological form but have a different meaning. For example, the suffix –er can be derivative (e.g. sell ⇒ seller) or inflectional (e.g. small ⇒ smaller). These types of morphemes are called homophonous.
Some words might seem to be composed of multiple morphemes, but in fact, they are not. This is why one has to consider form and meaning when identifying morphemes. For example, the word relate might seem to be composed of two morphemes, re- (prefix) and the word late, but this is not correct. These morphemes have no relationship with the definitions relevant to the word like “feel sympathy”, “narrate”, or “being connected by blood or marriage”. Furthermore, the length of the words does not determine if it has multiple morphemes or not. To demonstrate, the word Madagascar is long and it might seem to have morphemes like mad, gas, and car, but it does not. Conversely, small words can have multiple morphemes (e.g. dogs).
In natural language processing for Korean, Japanese, Chinese and other languages, morphological analysis is the process of segmenting a sentence into a row of morphemes. Morphological analysis is closely related to part-of-speech tagging, but word segmentation is required for these languages because word boundaries are not indicated by blank spaces.
The purpose of the morphological analysis is to determine the minimal units of meaning in a language or morphemes by using comparisons of similar forms—for example, comparing forms such as “She is walking” and “They are walking” rather than comparing either of these with something completely different like "You are reading". Thus, we can effectively break down the forms in parts and distinguish the different morphemes. Similarly, the meaning and the form are equally important for the identification of morphemes. For instance, agent and comparative morphemes illustrate this point. An agent morpheme is an affix like -er that transforms a verb into a noun (e.g. teach → teacher). On the other hand, –er can also be a comparative morpheme that changes an adjective into another degree of the same adjective (e.g. small → smaller). In this case, the form is the same, but the meaning of both morphemes is different. Also, the opposite can occur in which the meaning is the same but the form is different.
In generative grammar, the definition of a morpheme depends heavily on whether syntactic trees have morphemes as leaves or features as leaves.
Given the definition of a morpheme as "the smallest meaningful unit", nanosyntax aims to account for idioms where it is often an entire syntactic tree which contributes "the smallest meaningful unit." An example idiom is "Don't let the cat out of the bag" where the idiom is composed of "let the cat out of the bag" and that might be considered a semantic morpheme, which is composed of many syntactic morphemes. Other cases where the "smallest meaningful unit" is larger than a word include some collocations such as "in view of" and "business intelligence" where the words together have a specific meaning.
The definition of morphemes also plays a significant role in the interfaces of generative grammar in the following theoretical constructs;