In linguistics
Linguistics is the scientific study of human language. Linguistics can be broadly broken into three categories or subfields of study: language form, language meaning, and language in context....

, a morpheme is the smallest semantically
Semantics is the study of meaning. It focuses on the relation between signifiers, such as words, phrases, signs and symbols, and what they stand for, their denotata....

Meaning (linguistics)
In linguistics, meaning is what is expressed by the writer or speaker, and what is conveyed to the reader or listener, provided that they talk about the same thing . In other words if the object and the name of the object and the concepts in their head are the same...

 unit in a language. The field of study dedicated to morphemes is called morphology
Morphology (linguistics)
In linguistics, morphology is the identification, analysis and description, in a language, of the structure of morphemes and other linguistic units, such as words, affixes, parts of speech, intonation/stress, or implied context...

. A morpheme is not identical to a word
In language, a word is the smallest free form that may be uttered in isolation with semantic or pragmatic content . This contrasts with a morpheme, which is the smallest unit of meaning but will not necessarily stand on its own...

, and the principal difference between the two is that a morpheme may or may not
Bound morpheme
In morphology, a bound morpheme is a morpheme that only appears as part of a larger word; a free morpheme is one that can stand alone.Affixes are always bound. English language affixes are either prefixes or suffixes. E.g., -ment in "shipment" and pre- in "prefix"...

 stand alone, whereas a word, by definition, is a freestanding unit of meaning. Every word comprises one or more morphemes.
Every morpheme can be classified as either free or bound.