The longest axis in a crystal.

A reference direction for angular coordinates used in describing the directional characteristics of a transducer; it is usually an axis of structural symmetry or the direction of maximum response.

1. One of a set of perpendicular axes such that a quadratic function can be written as a sum of squares of coordinates referred to these axes.

2. For a conic, a straight line that passes through the midpoints of all the chords perpendicular to it.

3. For a quadric surface, the intersection of two principal planes.

One of three perpendicular axes in a rigid body such that the products of inertia about any two of them vanish.

See <<optic axis>>