1. two angles whose sum is a right angle;

The Collaborative International Dictionary of English v.0.48:

Complementary \Com`ple*men"ta*ry\, a. Serving to fill out or to complete; as, complementary numbers. [1913 Webster] Complementary colors. See under Color. Complementary angles (Math.), two angles whose sum is 90[deg]. [1913 Webster]WordNet (r) 3.0 (2006):

complementary angles n 1: two angles whose sum is a right angle