What is a teleconverter lens?

Answer 1

Or...

"Teleconverter lens" is also used to describe an accessory lens that you screw onto the front of a fixed-mount lens to give it more magnification. This would be used on a typical point and shoot camera and a "teleconverter" as described by the others above me would be used on an SLR.

Answer 2

a small lens that attaches to the back of a SLR lens which increases the focal length of the lens.

for example:
A standard 50mm lens will become 100mm when using a 2x teleconverter