RhymeZone: defocus definitions

Definitions of defocus:

verb: (transitive) To cause (a lens, or a beam of light or particles, etc.) to be out of focus.
noun: The process of going out of focus.
noun: (optics) The distance by which the focal point is in front of (positive defocus) or beyond (negative defocus) the image or target surface.

(Definitions from Wiktionary)