Idea of Lock Icon
The image is the basic rounded outline of a padlock. The padlock has a U-shaped part sticking out the top where you would loop it through the thing your are locking. Currently it is in the locked position. In the middle of the main, round, body of the lock is a keyhole where the pins of the key would go in pointing down.
This page contains the Lock vector icon, as well as variations of this icon in different visual styles, and related icons. All icons are in the flat vector style, however, differ by the line thickness, fill, and corner radius.