This is the first revision of this publication, which formalizes the work of the System Controls Subcommittee of the Power System Engineering Committee on the terminology for automatic generation control on electric power systems. The need for standard terminology in this field became apparent after a study of technical papers indicated that many authors were using different definitions for their terms. Requests for the standardization of terms were also received from the operating groups of the large interconnected power systems in this country and Canada. This is the first publication in English devoted specifically to the terminology used in automatic generation control.