The unitard is a body-length, single-piece garment, usually made of stretchy fabrics like Lycra, which closely adheres to the body. The earliest unitard styles were likely the wetsuit, which was worn for the very specific application of snorkeling and diving in colder temperatures. For all intents and purposes, a wetsuit is really a unitard, though it is made of rubber and is much bulkier than most unitards.

The catsuit also predates the unitard and is a form fitting one piece garment that gained popularity during the 1960s. These early unitards were popular on television adventure and spy shows like The Avengers. Today the catsuit is seen and worn by characters in a number of science fiction movies like X-Men. Early catsuits could be made of either stretchy fabric or leather. The leather catsuit is a very constricting outfit, and for women, often features a built-in corset. Michelle Pfeiffer has stated in the past that her decision not to play Catwoman again was largely based on having to fit into the very restrictive catsuit costuming.

Lycra, invented in the late 1950s, was not commonly available for several decades. By the 1970s, it was much easier to find. Rock stars, like Freddy Mercury and David Bowie, often wore unitards when performing. The unitard also migrated to the dance world, where many dancers found them infinitely preferable to the leotard and tights ensemble. Both male and female gymnastics stars may wear a unitard, and male and female ice skaters frequently use them for performance or practice.

For a while in the 1980s, unitards became extremely popular as everyday clothing, as opposed to athletic clothing, and might be worn alone, or with a big shirt or skirt. In health clubs all over the world, unitards were often the preferred dress code for aerobics instructors. Today it is quite common to see acrobats, gymnasts and a variety of other athletes perform in the unitard. In Las Vegas at burlesque shows women wear sheer unitards that add support but give the illusion of nudity.

The unitard in early versions always featured long sleeves. Some unitards with a tank top are now popular, and a little cooler to wear. Actually the tank top unitard seems to be inspired by the wrestling singlet, a one-piece garment with a tank top that ends just above the knee.