In grammar, a part of speech (also a word class, a lexical class, or a lexical category) is a linguistic category of words (or more precisely lexical items), which is generally defined by the syntactic or morphological behaviour of the lexical item in question. 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.

Tense can also make finer distinctions than simple past-present-future; past tenses for example can cover general past, immediate past, or distant past, with the only difference between them being the distance on the timeline between the temporal reference points. Such distinctions are not precise: an event may be described in the remote past because it feels remote to the speaker, not because a set number of days have passed since it happened; it may also be remote because it is being contrasted with another, more recent, past event. This is similar to other forms of deixis such as this and that.