head

In a phrase, the word that determines the nature of the phrase (in contrast, for instance, to any modifiers or determiners)--such as the noun in a noun phrase.

Examples and Observations:

• "The head of the noun phrase a big man is man, and it is the singular form of this item which relates to the co-occurrence of singular verb forms, such as is, walks, etc.; the head of the verb phrase has put is put, and it is this verb which accounts for the use of object and adverbial later in the sentence (e.g. put it there). In phrases such as men and women, either item could be the head."
  (David Crystal, A Dictionary of Linguistics and Phonetics, Wiley-Blackwell, 2003)



• "There are two main definitions [of head], one narrower and due largely to Bloomfield, the other wider and now more usual, following work by R.S. Jackendoff in the 1970s. 1. In the narrower definition, a phrase p has a head h if h alone can bear any syntactic function that p can bear. E.g. very cold can be replaced by cold in any construction: very cold water or cold water, I feel very cold or I feel cold. Therefore the adjective is its head and, by that token, the whole is an 'adjective phrase.'
  2. In the wider definition, a phrase p has a head h if the presence of h determines the range of syntactic functions that p can bear. E.g. the constructions into which on the table can enter are determined by the presence of a preposition, on. Therefore the preposition is its head and, by that token, it is a 'prepositional phrase.'"
  (P.H. Matthews, The Concise Oxford Dictionary of Linguistics, Oxford University Press, 1997)