Probably more of a suit as it is functional?
Technically in the movie world none of the things they wear are considered costume? Most are functional or can be considered more of a uniform like Shields suits. IMs is a functional suit, Thors battle armor or just typical Asgardian clothes, Caps looks functional and provides some protection to him, and same with BWs spandex. Even Hawkeyes dress can be considered as some protection of sorts.