The distance from the secondary nodal point of a lens to the secondary focal point. For a thin lens imaging a distant source, the focal length is the distance between the lens and the focal point.

Also Focus (noun).  The point toward which radiation converges or from which radiation diverges or appears to diverge.

Noun.  The point where rays of light meet which have been converged by a lens, giving rise to an image of the source. Verb: To adjust focal length for the clearest image.  Also Focal Point.

Construction in which the interior optical path is bent by mirrors mounted on corner blocks bolted into pre-aligned position, permitting compact packaging of a long laser cavity.

The number of light waves passing a fixed point unit of time, or the number of complete vibrations in that period of time.