Morpheme

The smallest unit of meaning in any language. Used by instructors and other educators in the field of reading and linguistics, a morpheme can range in size from a single letter (a, for example) to a multiple-letter word—e.g., brain. In turn, a word can consist of a single morpheme (a, brain) or several morphemes, as in baseball.
Morphemes do not have to be complete words. Prefixes and suffixes, for example, can seldom stand alone as independent words but nevertheless add meaning to words. In compound words with affixes that cannot stand alone, the central syllable that can stand alone as a word (control, for example, in uncontrolled) is called a free morpheme, while the affixes—un and ed—are bound morphemes, because any meaning they have is tied, or bound, to the free morphemes they modify.