In linguistics, a morpheme is the smallest grammatical unit in a language. In other words, it is the smallest meaningful unit of 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 freestanding. When it stands by itself, it is considered a root because it has a meaning of its own (e.g. the morpheme cat) and when it depends on another morpheme to express an idea, it is an affix because it has a grammatical function (e.g. the –s in cats to specify that it is plural). Every word comprises one or more morphemes. The more combinations a morpheme is found in, the more productive it is said to be.