1. One that rules; specifically; sovereign
2. A worker or a machine that rules paper
3. A smooth-edged strip (as of wood or metal) that is usually marked off in units (as inches) and is used as a straightedge or for measuring
4. A person who rules or commands.

In some application programs, such as word processors, an on-screen scale marked off in inches or other units of measure and used to show line widths, tab settings, paragraph indents, and so on. In programs in which the ruler is “live,” the on-screen ruler can be used with the mouse or with the keyboard to set, adjust, or remove tab stops and other settings.