In refrence I call it garb., but really I prefer to call it clothing., cause its what it is.
I think of a costume as a sort of one night inexpensively made type outfit.
A good muslin shirt and cotton pants and real leather boots to me is hardly a costume. Clothing., I would definitely say period clothing., you can run tumble fumble .,drink !!! fall down., work and even frolic in them., no costume is going to hold up to all this piratin' abuse.