In linguistics, a morpheme is the smallest semantic unit in a language. The field of study dedicated to morphemes is called morphology. A morpheme is not identical to a word, and the principal difference between the two is that a morpheme may or may not stand alone, whereas a word, by definition, is a freestanding unit of meaning. Every word comprises one or more morphemes.

Read more about Morpheme:  Morphological Analysis, Changing Definitions of Morpheme