If you're on a team, and everyone wears similar outfits, they're uniforms. The X-Men typically wear uniforms.
Forgive me, but since when? Training costumes aside, what unifying design elements have the standard X-Men teams had? Original X-Men graduation costumes? Nope. The All-New, all Different X-Men, not even a unifying X-buckle. The Australian era team? Nope. Current team? Nope. etc
On the other hand, the first X-factor did have a unifying design elements, but with radically different colour schemes.