An interesting debate that once and again tells me, how difficult communication really is. What is a costume?
To me (and most Europeans, I think) the term costume is used for a garment or garments intended for special occasions and the word is neutral loaded.
That will cover celebrations like weddings (white or black tie functions) as well as Halloween etc. Further it comprises what we describe as “national dress” or “national costume” (sorry).

Bear (Bear Kilts) used to have a pay off like “A kilt is a garment and not a costume”. What did he mean by that?
To me it always said that if a kilt is worn as a substitute to pants = trousers it is a garment; if it is used for weddings or the like (only) it is a costume (or part of a costume – full highland wear).

Most of you seem to have another idea of a costume. Some of you have even said that calling a kilt a “costume” would be an insult to a Scotsman – even if many, if not most Scotsmen probably wear their kilts as seldom as I wear “white tie”.
I won’t argue with you about definitions – English is not my mother tongue – but I realize how easy it is to insult someone without wanting to – just because of differences in the interpretation of terms such as “a costume”.

GG - A non-Scot who will wear his kilt as a garment.