A syntactic category is a type of syntactic unit that theories of syntax assume. The traditional parts of speech (e.g. noun, verb, preposition, etc.) are syntactic categories, and in phrase structure grammars, the phrasal categories (e.g. noun phrase NP, verb phrase VP, prepositional phrase PP, etc.) are also syntactic categories. Phrase structure grammars draw an important distinction between lexical categories and phrasal categories. Dependency grammars, in contrast, do not acknowledge phrasal categories (at least not in the traditional sense), which means they work with lexical categories alone. Many grammars also draw a distinction between lexical categories and functional categories. In this regard, the terminology is by no means consistent. The one opposition (lexical category vs. phrasal category) and the other opposition (lexical category vs. functional category) are orthogonal to each other. The category types just mentioned (lexical, phrasal, functional) should not be confused with grammatical categories (also known as grammatical features), which are properties such as tense, gender, etc.
Defining criteria 
At least three criteria are used in defining syntactic categories:
- The type of meaning it expresses
- The type of affixes it takes
- The structure in which it occurs
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.
Lexical categories vs. phrasal categories 
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:
In terms of phrase structure rules, phrasal categories can occur to the left of the arrow while lexical categories cannot, e.g. NP → D N. Traditionally, a phrasal category should consist of two or more words, although conventions vary in this area. X-bar theory, for instance, often sees individual words corresponding to phrasal categories. Phrasal categories are illustrated with the following trees:
The lexical and phrasal categories are identified according to the node labels, phrasal categories receiving the "P" designation.
Lexical categories only 
Dependency grammars do not acknowledge phrasal categories in the way that phrase structure grammars do. What this means is that the distinction between lexical and phrasal categories disappears, the result being that only lexical categories are acknowledged. The tree representations are simpler because the number of nodes and categories is reduced, e.g.
The distinction between lexical and phrasal categories is absent here. The number of nodes is reduced by removing all nodes marked with "P". Note, however, that phrases can still be acknowledged insofar as any subtree that contains two or more words will qualify as a phrase.
Lexical categories vs. functional categories 
Many grammars draw a distinction between lexical categories and functional categories. This distinction is orthogonal to the distinction between lexical categories and phrasal categories. In this context, the term lexical category applies only to those parts of speech and their phrasal counterparts that form open classes and have full semantic content. The parts of speech that form closed classes and have mainly just functional content are called functional categories:
- Lexical categories
- Adjective (A) and adjective phrase (AP), adverb (Adv) and adverb phrase (AdvP), noun (N) and noun phrase (NP), verb and verb phrase (VP), preposition and prepositional phrase (PP)
- Functional categories
- Coordinate conjunction (C), determiner (D), negation (Neg), particle (Par), preposition (P) and prepositional phrase (PP), subordinate conjunction (Sub), etc.
There is disagreement in certain areas, for instance concerning the status of prepositions. The distinction between lexical and functional categories plays a big role in Chomskyan grammars (Transformational Grammar, Government and Binding Theory, Minimalist Program), where the role of the functional categories is large. Many phrasal categories are assumed that do not correspond directly to a specific part of speech, e.g. agreement phrase (AgrP), focus phrase (FP), inflection phrase (IP), tense phrase (TP), etc. In order to acknowledge such functional categories, one has to assume that the constellation is a primitive of the theory and that it exists separately from the words that appear. As a consequence, many grammar frameworks do not acknowledge such functional categories, e.g. Head Driven Phrase Structure Grammar, Dependency Grammar, etc.
See also 
- Dependency grammar
- Grammatical category
- Lexical category (part of speech)
- Phrase structure grammar
- For the general reasoning behind syntactic categories, see Bach (1974:70-71) and Haegeman (1994:36).
- See Culicover (1982:8ff.).
- See for instance Emonds (1976:14), Culicover (1982:12), Brown and Miller (1991:24, 105), Cowper (1992:20, 173), Napoli (1993:169, 52), Haegeman (1994:38), Culicover (1997:19), Brinton (2000:169).
- See for instance Emonds (1976:12), Culicover (1982:13), Brown and Miller (1991:107), Cowper (1992:20), Napoli(1993:165), Haegeman (1994:38).
- For examples of grammars that draw a distinction between lexical and functional categories, see for instance Fowler (1971:36, 40), Emonds (1976:13), Cowper (1992:173ff.), Culicover (1997:142), Haegeman and Guéron (1999:58), Falk (2001:34ff.), Carnie (2007:45f.).
- Bach, E. 1974. Syntactic theory. New York: Holt, Rinehart and Winston, Inc.
- Brinton, L. 2000. The structure of modern English. Amsterdam: John Benjamins Publishing Company.
- Brown, K. and J. Miller. 1991. Syntax: A linguistic introduction to sentence structure, 2nd edition. London: UK: HarperCollins Academic.
- Carnie, A. 2007. Syntax: A generative introduction, 2nd edition. Malden, MA: Blackwell Publishing.
- Cowper, E. 1992. A concise introduction to syntactic theory: The government-binding approach. Chicago: The University of Chicago Press.
- Culicover, P. 1982. Syntax, 2nd edition. New York: Academic Press.
- Culicover, P. 1997. Principles and Parameters: An introduction to syntactic theory. Oxford University Press.
- Emonds, J. 1976. A transformational approach to English syntax: Root, structure-preserving, and local-transformations. New York: Academic-Press.
- Falk, Y. 2001. Lexical-Functional Grammar: An introduction to parallel constraint-based syntax. Stanford, CA: CSLI Publications.
- Fowler, R. 1971. An introduction to transformational syntax. New York: Barnes and Noblles Inc.
- Haegeman, L. 1994. Introduction to government and binding theory, 2nd edition. Oxford, UK: Blackwell.
- Haegeman, L. and J. Guéron. 1999. English grammar: A generative perspective. Oxford, UK: Blackwell Publishers.