Definition for FOCUS
From Biology Forums Dictionary
1) Focus refers to the degree to which light rays coming from any particular part of an object pass through the lens and reconverge at the same point on a frame of the film negative, creating sharp outlines and distinct textures that match the original object. This optical property of the cinema creates variations in depth of field -- through shallow focus, deep focus, and techniques such as racking focus.
2) The degree of sharpness and clarity in a film image. "Out of focus": the images are blurred and lack linear definition.