Merriam-Webster Collegiate Dictionary - equator

 noun  Etymology: Middle English, from Medieval Latin a~, literally, equalizer, from Latin aequare  Date: 14th century  1. the great circle of the celestial sphere whose plane is perpendicular to the axis of the earth  2. a great circle of the earth or a celestial body that is everywhere equally distant from the two poles and divides the surface into the northern and southern hemispheres  3.  a. a circle or circular band dividing the surface of a body into two usually equal and symmetrical parts  b. ~ial plane the ~ of a dividing cell  4. great circle