A morpheme is the smallest meaningful lexical item in a language. A morpheme is not necessarily the same as a word. The main difference between a morpheme and a word is that a morpheme sometimes does not stand alone, but a word, by definition, always stands alone. The field of linguistic study dedicated to morphemes is called morphology.
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In English, when a morpheme can stand alone, it is considered a root because it has a meaning of its own (such as the morpheme cat). When it depends on another morpheme to express an idea, it is an affix because it has a grammatical function (such as the –s in cats to indicate plurality).. However this definition is not universal and it does not apply to, e.g. Latin where many roots cannot stand alone. For instance, the Latin root reg- (king) must always be suffixed with a case marker: rex (reg-s), reg-is, reg-i, etc. In a language like Latin, a root can be defined as the main lexical morpheme of a word.