In linguistics, the head or nucleus of a phrase is the word that determines the syntactic category of that phrase. For example, the head of the noun phrase "boiling hot water" is the noun (head noun) "water".
Analogously, the head of a compound is the stem that determines the semantic category of that compound. For example, the head of the compound noun "handbag" is "bag", since a handbag is a bag, not a hand.
In grammar, the genitive case (abbreviated gen) is the grammatical case that marks a word, usually a noun, as modifying another word, also usually a noun—thus indicating an attributive relationship of one noun to the other noun. A genitive can also serve purposes indicating other relationships. For example, some verbs may feature arguments in the genitive case; and the genitive case may also have adverbial uses (see adverbial genitive).
The genitive construction includes the genitive case, but is a broader category. Placing a modifying noun in the genitive case is one way of indicating that it is related to a head noun, in a genitive construction. However, there are other ways to indicate a genitive construction. For example, many Afroasiatic languages place the head noun (rather than the modifying noun) in the construct state.