Plane, n.
 1. Geom. A surface, real or imaginary, in which, if any two points are taken, the straight line which joins them lies wholly in that surface; or a surface, any section of which by a like surface is a straight line; a surface without curvature.
 2. Astron. An ideal surface, conceived as coinciding with, or containing, some designated astronomical line, circle, or other curve; as, the plane of an orbit; the plane of the ecliptic, or of the equator.
 3. Mech. A block or plate having a perfectly flat surface, used as a standard of flatness; a surface plate.
 4. Joinery A tool for smoothing boards or other surfaces of wood, for forming moldings, etc. It consists of a smooth-soled stock, usually of wood, from the under side or face of which projects slightly the steel cutting edge of a chisel, called the iron, which inclines backward, with an apperture in front for the escape of shavings; as, the jack plane; the smoothing plane; the molding plane, etc.
 Objective plane Surv., the horizontal plane upon which the object which is to be delineated, or whose place is to be determined, is supposed to stand.
 Perspective plane. See Perspective.
 Plane at infinity Geom., a plane in which points infinitely distant are conceived as situated.
 Plane iron, the cutting chisel of a joiner's plane.
 Plane of polarization. Opt. See Polarization.
 Plane of projection. (a) The plane on which the projection is made, corresponding to the perspective plane in perspective; -- called also principal plane. (b) Descriptive Geom. One of the planes to which points are referred for the purpose of determining their relative position in space.
 Plane of refraction or Plane of reflection Opt., the plane in which lie both the incident ray and the refracted or reflected ray.