According to linguistic study, a morpheme is the smallest meaningful unit in a given language. This is the definition established in 1933 by the American linguist Leonard Bloomfield.
English Example:
The word "unbelievable" has three morphemes "un-", a bound morpheme, meaning "non-", "-believe-" a free morpheme, and "-able". "un-" is also a prefix, "-able" is a suffix. Both are affixes.
Types of morphemes:
Free morphemes like town, dog can appear with other lexemes (as in town-hall or dog-house) or they can stand alone, or "free". Allomorphs are variants of a morpheme, e.g. the plural marker in English is sometimes realized as /-z/, /-s/ or /-Iz/.
Bound morphemes like 'un'- appear only together with other morphemes to form a lexeme. Bound morphemes in general tend to be prefixes and suffixes.
Inflectional morphemes modify a word's tense, number, aspect, and so on.
Derivational morphemes can be added to a word to create (derive) another word: the addition of "-ness" to "happy", for instance > "happiness."
