plane, in mathematics, flat surface of infinite extent but no thickness. An example of a plane, or more exactly of a bounded portion of a plane, is the surface forming one face, or side, of a cube. A plane is determined, or defined, by any of the following: (1) three points not in a straight line; (2) a straight line and a point not on the line; (3) two intersecting lines; or (4) two parallel lines. Two straight lines in space do not usually lie in the same plane. For a given plane in space, a line can either lie outside and parallel to it, intersect the plane in a single point, or lie entirely in the plane; if more than one point of a straight line lies in the plane, then the entire line must lie in the plane.

*The Columbia Electronic Encyclopedia,* 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.

- Planes - The world's notable passenger, fighter, bomber, experimental, and spy planes
- Experimental and Spy Planes: Lockheed SR-71 Blackbird - The plane that literally grew several inches at Mach 3
- Planes: The Spirit of St. Louis - All about the Spirit of St. Louis
- Passenger Planes: Boeing 747 - They've flown more than 3.5 billion passengers, the equivalent of more than half the world's population
- Passenger Planes: Boeing 707 - The world's first successful commercial jetliner