Trekipedia

U.S.S. Challenger

Challenger (II) class.^[1] Prototype of the line. At least the fourth starship to bear the name, and the second to be the first of a class.

The class name is taken from The Star Trek Encyclopedia and was not used on screen. Though the ship has never appeared on screen or in the tie-in fiction, others of the class have; it must exist, given the tradition of naming classes for the first ship of the line.