Duality means a thing has two parts, or two sides, at the same time. It is a rather formal word for that “two-ness.” In science you will hear it about light, which can act like a wave and like a tiny bit of matter. In ordinary writing, people stretch it to a person or a place with two very different faces — warm in one room, hard in another. Though actually, that everyday use is not a science claim; it is just a way of saying “both, and they do not sit easily together.” You will meet it in essays more than in chat.

雙重性；二元性。物理課的「波粒二象性」是專門講法，日常比較常說雙重性。