In a triangle with a right angle (one corner of 90 degrees), the sine of one of the other angles is a fraction: take the side opposite that angle and divide it by the longest side — the side opposite the right angle, called the hypotenuse. People write it sin, so sin 30° = 1/2. You meet it in school trigonometry, and later in science: a sine wave is the smooth up-and-down curve this function draws. Though actually, the English word has a messy birth. Latin sinus meant a curve, a fold, a bosom. Medieval scholars used it to translate an Arabic maths word. The Arabic letters were written without vowels and looked like the word for “bosom,” but they stood for a different Arabic word meaning sine, which had come from a Sanskrit word for a bowstring (a chord of a circle looks like a string on a bow). So sine is something of a translation accident, and it is a doublet of sinus — same Latin parent, different life in English. The job-word sinecure uses a different Latin sine, meaning “without.” That is not this word.