A circle with a vertical line at the top, the universal power symbol. You'd find this on a device's settings menu to turn the device off or restart it, or on a remote control to toggle the main power, though it could also appear as a standby button on an app's home screen.
Part of the
Multimedia Elements SVG collection,
in Technology.
Download the Power vector as a ready-to-edit .svg file —
it scales to any size without losing quality and can be recoloured in any
vector editor or straight in CSS.