A "kit" lens is a lens included with a camera body at time of purchase.
Many DSLR cameras today come with a lens when purchased. This lens is often referred to as the "kit lens". The most common kit lens for entry and mid-level DSLR's is the 18-55mm lens, however professional grade cameras often include higher quality, professional grade kit lenses. An example of this would be the Canon 5D Mark II, which can be purchased with either the 24-105mm L IS series lens, or in some cases 24-70mm L series lens.