A finitely generated discontinuous group of linear fractional transformation acting on a domain in the Complex Plane.
References
Iyanaga, S. and Kawada, Y. (Eds.). Encyclopedic Dictionary of Mathematics. Cambridge, MA: MIT Press,
p. 425, 1980.
Kra, I. Automorphic Forms and Kleinian Groups. Reading, MA: W. A. Benjamin, 1972.