'Principal axis' definitions:

Definition of 'principal axis'

noun

A line that passes through the center of curvature of a lens so that light is neither reflected nor refracted; "in a normal eye the optic axis is the direction in which objects are seen most distinctly" [syn: principal axis, optic axis]

Definition of 'Principal axis'

From: GCIDE

Principal \Prin"ci*pal\, a. [F., from L. principalis. See Prince.]
1. Highest in rank, authority, character, importance, or degree; most considerable or important; chief; main; as, the principal officers of a Government; the principal men of a state; the principal productions of a country; the principal arguments in a case. [1913 Webster]
Wisdom is the principal thing. --Prov. iv. 7. [1913 Webster]
2. Of or pertaining to a prince; princely. [A Latinism] [Obs.] --Spenser. [1913 Webster]
Principal axis. See Axis of a curve, under Axis.
Principal axes of a quadric (Geom.), three lines in which the principal planes of the solid intersect two and two, as in an ellipsoid.
Principal challenge. (Law) See under Challenge.
Principal plane. See Plane of projection (a), under Plane.
Principal of a quadric (Geom.), three planes each of which is at right angles to the other two, and bisects all chords of the quadric perpendicular to the plane, as in an ellipsoid.
Principal point (Persp.), the projection of the point of sight upon the plane of projection.
Principal ray (Persp.), the line drawn through the point of sight perpendicular to the perspective plane.
Principal section (Crystallog.), a plane passing through the optical axis of a crystal. [1913 Webster]