focal length

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The distance of a lens or mirror to its focal point.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun optics The distance at which something (e.g. a lens, mirror etc.) is in focus; in air it is the distance along the optical axis from a "principal plane" of the lens to one of the lens's focal points.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the distance from a lens to its focus