A syntactic category is a syntactic unit that theories of syntax assume. Word classes, largely corresponding to traditional parts of speech (e.g. noun, verb, preposition, etc.), are syntactic categories. In phrase structure grammars, the phrasal categories (e.g. noun phrase, verb phrase, prepositional phrase, etc.) are also syntactic categories. Dependency grammars, however, do not acknowledge phrasal categories (at least not in the traditional sense).

Word classes considered as syntactic categories may be called lexical categories, as distinct from phrasal categories. The terminology is somewhat inconsistent between the theoretical models of different linguists. However, many grammars also draw a distinction between lexical categories (which tend to consist of content words, or phrases headed by them) and functional categories (which tend to consist of function words or abstract functional elements, or phrases headed by them). The term lexical category therefore has two distinct meanings. Moreover, syntactic categories should not be confused with grammatical categories (also known as grammatical features), which are properties such as tense, gender, etc.

At least three criteria are used in defining syntactic categories:

For instance, many nouns in English denote concrete entities, they are pluralized with the suffix -s, and they occur as subjects and objects in clauses. Many verbs denote actions or states, they are conjugated with agreement suffixes (e.g. -s of the third person singular in English), and in English they tend to show up in medial positions of the clauses in which they appear.

The third criterion is also known as distribution. The distribution of a given syntactic unit determines the syntactic category to which it belongs. The distributional behavior of syntactic units is identified by substitution. Like syntactic units can be substituted for each other.

Additionally, there are also informal criteria one can use in order to determine syntactic categories. For example, one informal means of determining if an item is lexical, as opposed to functional, is to see if it is left behind in "telegraphic speech" (that is, the way a telegram would be written; e.g., Pants fire. Bring water, need help.)

The traditional parts of speech are lexical categories, in one meaning of that term. Traditional grammars tend to acknowledge approximately eight to twelve lexical categories, e.g.

The lexical categories that a given grammar assumes will likely vary from this list. Certainly numerous subcategories can be acknowledged. For instance, one can view pronouns as a subtype of noun, and verbs can be divided into finite verbs and non-finite verbs (e.g. gerund, infinitive, participle, etc.). The central lexical categories give rise to corresponding phrasal categories: