The Collaborative International Dictionary of English v.0.48:
Headroom \Head"room`\ (-r[=oo]m`), n. (Arch.) See Headway, 2. [Mostly Brit.] [1913 Webster +PJC]WordNet (r) 3.0 (2006):
headroom n 1: vertical space available to allow easy passage under something [syn: headroom, headway, clearance] 2: the capacity of a system to reproduce loud sounds without distortion [syn: headroom, dynamic headroom]