In an X-bar notation, the head of a category to be expanded is always shown by the notation. It is traceable from the rule format that the head of a Noun Phrase is the Noun, of an Adjective Phrase the Adjective and so on.
- Bauer (2004), Pag. 171

The head of a phrase is the subcategorizand, i.e. the item which is subcategorized for the occurrence of other elements in the phrase. [...] The head of a phrase is the morphosyntactic locus, either in that it bears the overt inflectional marking which is typical of the phrase as a whole, or, more subtly, in that it is the item from which morphosyntactic information "percolates" to the entire construction. [...] The head of a phrase is the governor, i.e. the element which determines the morphological shape of some other element in the phrase. [...] The head of the phrase is the distributional equivalent of the whole phrase [...] The head of a phrase is an obligatory constituent in the phrase.
- Bauer (2004), Pag. 172