For me, it becomes costume when you're wearing it not because you like it or it's comfortable, but because how others perceive you is all that matters. It's costume when some women insist on wearing a fur coat at Summer events "Because it's vintage" (and often because it's an excuse to wear newer fur in a setting where they won't be questioned on it), despite the temperature making it wildly inappropriate. It's costume when some men wear spectators / co-respondents in Winter. It's costume when men wear a hat.... and fail to remove it when appropriate (i.e. during dinner, at a dance, or similar). It's costume when you spend an hour putting on your best suit to go do the groceries.