Head noun

In an expression or string of words, a head noun is the main noun or the noun which determines what main kind of "thing" the expression is about. The other words in the expression go on to qualify or give more information about the main thing. Hence in "tree of life", the head noun is "tree". What kind of thing is it? It's a tree. The other noun "life" gives more information about the tree. Similarly, in "hand bag", the head noun is bag. What kind of thing is it? It's a bag. "Hand" is just there to give more information about what kind of bag.