Short for cotangent; the trigonometric function that is the reciprocal of the tangent, or the ratio of the adjacent side to the opposite side in a right triangle.
From 'cotangent', shortened for convenience in mathematical writing. 'Co-' here means 'complementary' (from Latin 'co-' originally meaning 'with'), and 'tangent' comes from Latin 'tangens' meaning 'touching'.
Medieval mathematicians noticed that certain trigonometric ratios were reciprocals of others, leading them to create 'complementary' functions—'cosine' and 'cotangent' both have that 'co-' prefix meaning they're the flip-side versions of sine and tangent.
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