The triangle (as distinct from the basic geometric shape) is a musical instrument that is, as its name suggests, in the shape of a triangle. Come to think of it, it isn't really that distinct from the shape. It's hardly a different thing at all. They both have the same name. If I overheard you telling your wife about a triangle, I wouldn't know which kind of triangle you were referring to, unless a context was supplied. Your wife would know, of course. She always knows.