A leotard is a skin-tight one-piece garment (very similar to the one-piece swimsuit) which covers the body/torso of the wearer, and sometimes the arms, but leaves the legs uncovered. Leotards with “short” length legs are called unitards and ones with long legs are called catsuits.
Popularised by Jules Léotard, leotards help emphasis the body of the wearer and is ideal for dance (especially ballet) as it allows teachers and choreographers to see the “line” of the body. They can also be worn as aerobic exercise wear and on stage.
Leotards can be worn by women or men (although female specific leotards tend to have bra-shelves and a higher-leg cut, mens leotards tend to have a slightly lower cut leg and lower cut front).