In "a head of cabbage," the word "head" refers to the rounded, compact mass of leaves that make up the cabbage. It is a common term used to describe the whole or entire cabbage in its natural form. So, while "head" indirectly implies "whole" or "entire" in this context, its primary meaning is the shape or structure of the cabbage.

It's a fancy counting word. You might see it in a recipe. 1 head of lettuce. Just a whole lettuce.