Trivial comes from the Latin trivium, the place where three roads meet. Whatever was said there was heard by everyone passing, so trivialis came to mean common, ordinary. Medieval schools borrowed the same word for their first three subjects, grammar, logic and rhetoric, the ones every student had to get through before the harder four of the quadrivium, and in fifteenth-century English “trivial” meant belonging to those basics. So the word for what doesn’t matter began as the word for what everyone stands on. Mathematicians kept a little of the old sense: a trivial solution is not a wrong one, only one so plainly true that nobody gets credit for finding it. Perhaps that is the better definition. A thing is trivial not when it is unimportant, but when it is so widely shared that nobody remembers learning it.