Two colours from opposite sides of the colour wheel — a classic high-contrast pairing.
Hues separated by 180° on the hue wheel.
Complementary pairings are high hue-contrast, not necessarily high lightness-contrast — the two colours can still be similar in lightness.
Blue and orange, or red and green, are complementary pairs.
Complementary pairings are one of the calculated categories on every What Goes With This Colour? result.