The receptive, dark, yielding half of a pair that needs both. Yin is not lesser than yang; neither exists alone.
Chinese 陰 (yīn), literally "shade" or "the shaded side of a hill"; paired from antiquity with 陽 (yáng), the sunlit side, to name receptive, dark and yielding qualities that this cosmology treats as complementary rather than lesser — no yin without yang, and no yang without yin.
Receptivity and yielding, named as half of a pair rather than as a whole thing on its own — the other half is not optional. Neither side outranks the other; a pairing like this only exists as a pairing.