Real Dictionary

morpheme
countable noun: A morpheme is the smallest unit of meaning in a language. The words 'the', 'in', and ' girl' consist of one morpheme. The word 'girls' consists of two morphemes: 'girl' and 's'.

morpheme in American English
the smallest meaningful unit or form in a language: it may be an affix (Ex.: un- in undo or -er in doer) or a base (Ex.: do in undo)
noun: any of the minimal grammatical units of a language, each constituting a word or meaningful part of a word, that cannot be divided into smaller independent grammatical parts, as the, write, or the -ed of waited

morpheme in British English
noun: a speech element having a meaning or grammatical function that cannot be subdivided into further such elements