In grammar, the smallest unit of language that carries meaning is a morpheme. A morpheme may be an entire word, such as "cat," or only part of a word, such as the plural ending "-s" in "cats."
Morphemes are studied in morphology, the branch of linguistics concerned with word structure. A free morpheme can stand alone as a word, while a bound morpheme must attach to another form. Prefixes such as "un-" and suffixes such as "-ness" are common bound morphemes in English.
A morpheme is not the same as a phoneme, which is the smallest sound unit that can distinguish meaning but does not itself carry meaning. It also differs from a grapheme, a written symbol, and a lexeme, an abstract vocabulary item. Some morphemes have no spoken or written form in particular contexts; linguists may call these zero morphemes.