RU'LER, noun

1. One that governs, whether emperor, king, pope or governor; any one that exercises supreme power over others.

2. One that makes or executes laws in a limited or free government. Thus legislators and magistrates are called rulers.

3. A rule; an instrument of wood or metal with straight edges or sides, by which lines are drawn on paper, parchment or other substance. When a ruler has the lines of chords, tangents, sines, etc. it is called a plane scale.