Every word in the language belongs to some lexical category, that is, instances some part of speech. Thus, `dog', `brain', and `syllogism', for example, all belong to the lexical category `noun'.

In grammar, a lexical category (also word class, lexical class, or in traditional grammar part of speech) is a linguistic category of words (or more precisely lexical items) that are usually defined by their particular syntactic or morphological behaviours. Common linguistic categories include noun and verb, among others. There are open word classes, which constantly acquire new members, and closed word classes, which acquire new members infrequently if at all.