in geometry, denotes a plain surface, or one that lies evenly between its bounding lines: and as a right line is the shortest extension from one point to another, so a plane surface is the shortest extension from one line to another.

In astronomy, conics, &c. the term plane is frequently used for an imaginary surface, supposed to cut and pass through solid bodies; and on this foundation is the whole doctrine of conic sections built. See Astronomy, Conic Sections, &c.

In mechanics, planes are either horizontal, that is, parallel to the horizon, or inclined thereto. See Mechanics.

The determining how far any given plane deviates from an horizontal line, makes the whole business of levelling. See the article Levelling.

In optics, the planes of reflection and refraction, are those drawn through the incident and reflected or refracted rays. See Optics.

In perspective, we meet with the perspective plane, which is supposed to be pellucid, and perpendicular to the horizon; the horizontal plane, supposed to pass through the spectator's eye, parallel to the horizon; the geometrical plane, likewise parallel to the horizon, wherein the object to be represented is supposed to be placed, &c. See Perspective.

The plane of projection in the stereographic projection of the sphere, is that on which the projection is made, corresponding to the perspective plane. See Projection.

joinery, an edged tool or instrument for parting and shaving of wood smooth.—It consists of a piece of wood very smooth at bottom, as a block or shaft; in the midst of which is an aperture, through which a steel edge, or chisel, placed obliquely, passes; which, being very sharp, takes off the inequalities of the wood along which it slides.

Plane-tree, in botany. See Platanus.