Subtraction is the taking of one number away from another — the maths operation written with a minus sign: 10 − 4 = 6. It is one of the four basic operations children learn (addition, subtraction, multiplication, division), and in a classroom it is the plain, ordinary word: "We're doing subtraction today." It names the process, not the answer. The answer to a subtraction is called the difference. And note that it is normally uncountable when you mean the skill or the operation — "he's good at subtraction" — but countable when you mean one particular sum on the page: "there are twenty subtractions on this worksheet." Outside maths it appears now and then in a more careful, written way, meaning the removing of something from a whole: "the subtraction of tax from your salary." That use is formal; in everyday talk people just say taking away, deducting, or knocking off. There is also a slightly literary use where writers talk about a life or a scene being defined by subtraction — by what has been removed rather than added — but a student will meet that rarely, and only in reading.

減法；相減（數學課本上就叫「減法」，答案叫「差」）