A presumption is something you assume to be true without having proof. When you make a presumption, you're taking something for granted based on what seems likely or what usually happens, rather than on facts you know for sure.

If your teacher announces a pop quiz and you presume it will be multiple choice because that's the format she usually uses, you're making a presumption. You might be right, but you don't actually know until you see the quiz. If you see dark clouds gathering and presume it's going to rain, that's another presumption: a reasonable guess based on experience, but not certain knowledge.

The word often appears in law, where it has special importance. The presumption of innocence means courts must assume someone is innocent until proven guilty. This presumption protects people from being punished without evidence.

Sometimes presumptions turn out wrong, which is why people warn against “jumping to conclusions.” If you presume your friend is angry because she seems quiet, you might discover she's actually just tired or focused on something else. Making presumptions is natural and often helpful, but the wise approach is to stay open to being wrong. When you presume something, you're making your best guess with incomplete information.