It is a circle with a horizontal line directly in the middle of the circle. The horizontal line stops slightly before reaching the edge of the circle so no part of the line actually touches the circle itself.
This Minus icon has Android KitKat style
. If you use the icons for Android apps, we recommend using our latest Material Icons icon pack (please check our sidebar on the right). Otherwise, it's a solid icon pack that looks good in small resolutions, either on the web or in print. It's characterized by thick lines, smooth corners, and is optimized for 24x24 pixels.