The fact that two or more things happen in the same period of time, or belong to the same stretch of history. Writers use this noun when they want to name that same-time-ness as a thing in itself: the contemporaneity of two painters, or of a war and a book written during it. It is rare and very formal — almost only in history, art writing, and university work. Ordinary English almost never needs it. People say they lived at the same time, or they were contemporaries. Though actually contemporary, the everyday cousin, also means of now, modern, so students sometimes stretch contemporaneity toward being up to date. In careful use it still means same period, not fashionable. You will not hear this word in the shop or on the bus.

同時代性（學術用語；不是「當代感」）