head. The term comes from dependency grammar, where grammatical constituents are taken to have a head word, which determines the central properties of the constituent, and other subsidiary constituents. E.g., the nominal properties of a noun phrase are due to its head word being a noun, and subsidiary constituents, i.e. dependencies, are determiner, adjective, prepositional phrase, relative clause. When a noun. e.g. "man", occurs with such a dependent, e.g. a determiner "the", within a noun phrase, e.g. "the man", one says this dependency of the head has been satisfied.
There is lots of disagreement about which part of a grammatical constituent is the head (if any) and which parts are dependents.