A speedsuit is an item of unisex exercise attire or an industrial uniform used when quick clothing changes are necessary. It is either a single piece of clothing which tightly fits the torso and, optionally, varying amounts of the arms and legs; overall, it is similar to a leotard, though intentionally made especially tight and constricting to hug the body for varying purposes of warmth (when used in snowboarding or skiing) and hydrodynamics (when used in swimming and other water sports), or it is a tight fitting collared jumpsuit similar to coveralls.
In popular culture 
A running gag in The Venture Bros. episode "Hate Floats" is that Dr. Venture wants to buy his son a speed suit for his birthday. A gag in the episode revolves around Dr. Venture's comedic assertion that speedsuits are meant to be worn with the crotch tailored so that it hangs six inches below the bottom of the natural inseam. Superscientists like Dr. Venture do not have time to choose each day's clothing, in the imagined reality of the show, so the use of a uniform that is simple to put on and take off is vitally important.
See also