is another term for Gaussian Optics
A parfocal lens stays in focus when the magnification / focal length changes.
Some amount of defocussing is inevitable, but the amount is so small that it is considered as insignificant.
In microscopy parfocal lenses stay in focus when the magnification is changed.
The image stays focused when the magnification is changed from 40x to 10x
Most bright field microscoped are parfocal.
Motor-Zoom lenses are actually varifocal. When you change the magnification, also the focus changes. However the focus motor can work in parallel to the focal length motor, so for the user the lens seems to be parfocal.