Zentai is a term for skin-tight garment

Skin-tight garment

A skin-tight garment is a garment that is held to the skin by elastic tension. Many skin-tight garments are also one-piece garments.In athletics and performance applications, skin-tight garments—or 'skinsuits' provide protection from injury from dancing, gymnastics, swimming, cycling, skating,...

s that cover the entire body. The word is a contraction of zenshin taitsu (全身タイツ) ("full-body tights"). Zentai is most commonly made using nylon

Nylon

Nylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...

/spandex

Spandex

Spandex or elastane is a synthetic fibre known for its exceptional elasticity. It is strong, but less durable than natural Latex, its major non-synthetic competitor. It is a polyurethane-polyurea copolymer that was co-invented in 1959 by chemists C. L. Sandquist and Joseph Shivers at DuPont's...

 blends, but other materials such as cotton and wool are used as well.

There are several variations based on the Zentai suit including mummy bag (similar to a sleepsack) and hybrid suits consisting of either a single leg and arms, or separate legs and no arms.
Some companies have tried to create mainstream brands of the suits, by dropping the traditional name; in particular, examples include RootSuit or Superfan Suit in the United States and Bodysocks or Morphsuits

Morphsuits

Morphsuits is a company based in Edinburgh, Scotland. It distributes branded spandex costumes, based on the existing zentai concept. As of May 2011, it claims to be the world's largest fancy dress brand. The company offers at least 50 different designs....

 in the United Kingdom.