Spandex is a type of stretchy fabric that is often used in clothing such as leggings or athletic wear. It allows the fabric to stretch and fit closely to the body. Full definition
So again, having Samus parade around in spandex in DOA isn't anything new, because she does that in Smash Bros, Other M, you name it (well, actually not in the Prime games so much). (nintendoworldreport.com)