What do you understand by the terms 'focus' and 'principal focus' in the context of lenses ?

i) Principal Focus (F) : When rays of light are incident on a lens in a direction parallel to the principal axis.The point on principal axis at which all light rays converge (or appear to diverge) is called principal focus.
ii) Focus : If the parallel beam of light were incident making some angle with the principal axis, the refracted rays would converge or appear to diverge from a point on principal axis is called focus. It is just a random point where some parallel rays meet, it might be at the principal focus or otherwise. It also need no be placed on the principal axis of the lens.