Originally Posted by ic-racer
"Focus" has a specific definition as dictated by the physics of light. Perhaps you mean "what is it to be 'in-focus?'" That is a subjective concept.
I would say the the term 'in focus' has a specific optical definition; when 'in focus' a spot would have the smallest circle of confusion.
The term 'focus' by itself doesn't seem to have a specific meaning.