Many years ago, in the good old days of films; SLR were sold with a standard lens of 50 or 55 mm range. The word "kit lens" was not invented yet :)
Wikipedia defined kit lens as:
"A "starter" lens which can be sold with an interchangeable-lens camera such as a single-lens reflex camera. The kit consists of the camera body, the lens, and various accessories usually necessary to get started in SLR photography. A kit lens can be sold by itself; ie; not in a kit.
It is generally an inexpensive lens priced at the lowest end of the manufacturer's range so as to not add much to a camera kit's price.
Originally kit lenses were of normal focal length; more recently kit lenses tend to be inexpensive zoom lenses that range from medium wide angle to mid telephoto for added versatility. Prime lenses are generally faster (smaller f-number), but the change to zoom lenses has usually means that a slower (higher f-number) lens is used.
This is because in most cases prime lenses are faster than comparably priced zoom lenses. However in most cases the inclusion of an inexpensive zoom lens is to maintain a low entry price and maximize usability for the beginner photographer."
Read that 4th paragraph again ;)