What makes the difference between "clothing" and "costume" in many cases is the attitude of the wearer.

When I got married, myself and all the groomsmen were in identical outfits: kilt, hose, flashes, black shoes, sgian dubh, sporran, belt, tux shirt, black bow tie, and black Argyle jackets. I had been wearing the kilt regularly for many years at that point -- most of my groomsmen had never worn a kilt before in their lives.

Though we were all dressed identically, several people (my wife, her brother, etc) made the comment that they all looked like they were wearing costumes, while I just looked like I was wearing clothes.

Chalk it up to the fact that I was comfortable in the kilt and carried myself in a comfortable and easy manner. The other guys were excited about wearing somethign different and new and were more self conscious about the fact that they were kilted. It's nothing conscious that they did differently, just an inner attitude.

So I would say that if you are wearing the kilt in a manner, or with a purpose, that says, "Look at me, I'm kilted!" then it is a costume you have put on. If you wear the kilt as another form of clothing (either every day, or just on special occasions) then it is clothing.

I had a recent post in my blog that touched on this a bit. I was making the point that if you wear the kilt as clothing, not costume, avoid the danger of having every last bit of your outfit scream 'Scotland!' and let the kilt speak for itself. I brought up the instance of the man I saw in a very nice kilt, with hose embroidered with thisles on the cuff, a rugby shirt that said "Scotland" across the breast, and a ball cap that also said "Scotland" on it. His whole ensemble looked like he got up and said "I'm going to put on all my Scottish stuff today!" He looked like a walking advert for the Scottish Tourist Board. That was a costume.

Had his shirt and hat not been blazoned with the country's name, the outfit would have been just fine.

Matt