In morpheme-based morphology, a morpheme is the smallest linguistic unit that has semantic meaning.
In spoken language, morphemes are composed of phonemes (the smallest linguistically distinctive units of sound), and in written language morphemes are composed of graphemes (the smallest units of written language).

The concept morpheme differs from the concept word, as many morphemes cannot stand as words on their own. A morpheme is free if it can stand alone, or bound if it is used exclusively alongside a free morpheme. Its actual phonetic representation is the morph, with the morphs representing the same morpheme being grouped as its allomorphs.

; ''English example:''
The word "unbreakable" has three morphemes: "un-" (meaning ''not x''), a bound morpheme; "-break-", a free morpheme; and "-able", a bound morpheme. "un-" is also a prefix, "-able" is a suffix. Both are affixes.

The morpheme plural-s has the morph "-s", s, in ''cats'' (kæts), but "-es", iz, in ''dishes'' (diʃɪz), and even the voiced "-s", z, in ''dogs'' (dogz). These are the allomorphs of "-s". It might even change entirely into -ren in ''children''.

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, i.e. "free".

Bound morphemes (or affixes) like "un-" appear only together with other morphemes to form a lexeme. Bound morphemes in general tend to be prefixes and suffixes. Unproductive, non-affix morphemes that exist only in bound form are known as cranberry morphemes, from the "cran" in that very word.

Inflectional morphemes modify a word's tense, number, aspect, and so on (as in the ''dog'' morpheme if written with the plural marker morpheme ''s'' becomes ''dogs'').

Derivational morphemes can be added to a word to create (derive) another word: the addition of "-ness" to "happy," for example, to give "happiness."

Allomorphs are variants of a morpheme, e.g. the plural marker in English is sometimes realized as -z, -s or -.

Morphological analysis

In natural language processing for Japanese, Chinese and other languages, morphological analysis is the process of segmenting a given sentence into a row of morphemes. It 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. Famous Japanese morphological analysts include Juman and ChaSen.