Here is a description of the basic Tadpole.
The term Tadpole Trike is related to the view from above. A tadpole recumbent trike with the two wheels at the front and single drive wheel at the rear takes on a little of the shape of a Tadpole compared to the longer narrow front style of a Delta trike.
Tadpole Trike Anatomy
Delta and Tadpole Trikes
There are in the simple terms, two styles of recumbent trikes, the Tadpole and the Delta: