‘Parfocal’ is a term related to zoom lenses, and indicates that the zoom is constructed in such a way that the focusing distance remains stable while zooming.
A zoom lens being parfocal’ is useful in two kinds of situations:
Firstly, in still photography, a parfocal zoom allows you to first focus, and then still change your mind about the focal length (in other words: amount of zoom), without having to refocus.
Secondly, in videography, it allows you to zoom in/out while “the camera is rolling” without losing focus.
Furthermore, parfocal lenses tend to have less focus breathing.
The opposite of a parfocal (zoom) lens is a varifocal zoom lens. In fact, some consider that only parfocal lenses should be called ‘zooms’, whereas varifocal lenses should be referred to as ‘variable focal length” lenses.
While parfocal zoom lenses seem to have a lot going for them, there are a trade-off as parfocal zooms tend to be bigger, heavier and more expensive.
If you want to test your zoom for whether it is parfocal, keep in mind that parfocal and varifocal are the ends of a spectrum, and while most modern still photography zooms evidence decent levels of parfocal behavior (i.e. the focus is not waay off after zooming), very few zoom lenses are truly parfocal without having been expressly designed for it.
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