In linguistics, a noun class is a particular category of nouns. A noun may belong to a given class because of characteristic features of its referent, such as sex, animacy, shape, but counting a given noun among nouns of such or another class is often clearly conventional. Some authors use the term "grammatical gender" as a synonym of "noun class", but others use different definitions for each (see below). Noun classes should not be confused with noun classifiers.