Morpheme is the smallest unit of languages. The minimal language unit has both: the content(meaning) and expression(form). Oxford dictionary dives such definition: the morpheme is any configuration of phonological units within a word which has either grammatical or lexical meaning.
The real segment of a word is called a "morph". It is the speech counterpart of the morpheme. The morph is the smallest meaningful succession of phones which cannot be divided into any other meaningful units. Another term connected with the notions of the morph and morpheme is allomorph. Allomorphs are morphs representing a certain morpheme.
Types of morphemes:
- according to the form of expression: zero expresses a grammatical meaning, but has no explicit form and isn't a segment of the word; positive expresses - has a positive explicit form of expression
- according to the role: free -can be used separately; semi-bound - can be free or bound; bound - don't function separately
- according to the position: roots are the lexical morphemes of the word, common lexical elements in a word family, it is a free morpheme; stem is a word without its grammatical morphemes, it may consist of a roof alone or a roof + word building suffix; affix is a grammatical morpheme, which are always used with a roof and never function separately