Circuit return - Ground, general symbol.
also:A direct conducting to the earth or body of water that is a part thereof.
A conducting connection to a structure that serves a function similar to that of an earth ground (that is, a structure such as a  frame of air, space, or land vehicle that is not conductively connected to earth).
(in English) (1993)  315-1975 - IEEE Standard American National Standard Canadian Standard Graphic Symbols for Electrical and Electronics Diagrams (Including Reference Designation Letters), IEEE, p. 50  DOI:  10.1109/IEEESTD.1993.93397. ISBN:  0738109479.
Michel Bakni