Complement domain

In generative syntax, a complement domain is a notion in checking theory. The complement domain is the subset of a domain reflexively dominated by a complement.

Example
In the example, YP is the complement domain of X (and H).

XP1 /\     /  \     UP  XP2 /\      /  \     ZP1    X'    /\      /\ / \    /  \  WP  ZP2  X1  YP          /\ / \        H   X2

Link
Utrecht Lexicon of Linguistics