In theory, a morpheme is the smallest contrastive unit of grammar in a given language. The notion of morpheme shouldn’t be mistaken with the word. Several morphemes can be used for one word. The french word injustement (“unfairly”) can thus be broken down to three morphemes: in– (negative prefix), juste (adjective stem, holding the main signification), –ment (adverbial derivation suffix).

On the other hand, certain words cannot be decomposed such as “steam” or “mosquito” in English. These words are only made out of one morpheme.

Generally speaking, there are two main types of morphemes: grammatical and lexical. Grammatical morphemes belong to a “closed” category, whereas lexical morphemes belong to an “open” category. For instance, if there are 2 numbers, 2 genders and 6 persons in English, it’s possible to assess the morphemes denoting those notions, but it would almost be impossible to draw up the inventory of all the nominal and verbal roots of the English language.