Original: Two angles that sum to a straight angle (180 °) are called supplementary angles.
Simplified: A straight angle (or straight line) is an angle equal to 180 °. Supplementary angles are two angles with the sum equal to 180 °. Two angles that sum to one right angle (90 °) are called complementary angles. Two angles that sum to one full circle (360 °) are called explementary angles or conjugate angles.