"Head height" refers to the level at which something is positioned in relation to a person's head. It describes the height at which an object or obstacle is typically aligned with someone's head when they are standing. Full definition
The light buttons on the seat control panel control the reading light (at head height just above the headphone hook).... (travelingformiles.com)
Or at least the top of half of it did — the projected light needs to be above head height so it's not continually blocked by passing visitors. (community.bowdoin.edu)
It is very lightweight and has a maximum head height of 40 inches. (strollerlab.net)