This type of power icon consists of a circle form that represents zero and a vertical line which represents one. Zero and one standing for on and off, which derives from the binary system. In this form it should be used on buttons that switch a device between on and fully off states. The same symbol has a similar variation where the vertical line is partially within a broken circle in which case it should be used on buttons that toggle a device between on and standby states.
The symbol is part of the International Electrotechnical Commission standard where it is coded as IEC 5010.
The icon appears in the 1973 edition of the IEC standard.