Headroom
The space between the top of a subject's head and the upper edge of the frame.
In depth
Too much headroom drops the subject to the bottom of the frame and makes the picture feel unbalanced; too little feels cramped, though a deliberate crop into the top of the head is a recognised device in tight portraiture. The convention places the eyes roughly on the upper third, which sets headroom automatically.
Examples in practice
A head-and-shoulders portrait with the eyes on the upper third leaves a small, comfortable gap above the hair.