Varifocal Lens
A zoom lens that does not hold focus when the focal length changes, requiring refocusing after every zoom.
In depth
Almost every stills zoom is varifocal, because autofocus refocuses instantly and the cost saving over a true parfocal design is considerable. The consequence only becomes obvious in manual use and in video, where zooming mid-shot drifts out of focus unless the operator compensates.
Examples in practice
Zooming a stills lens from 24 mm to 70 mm during a video clip leaves the subject visibly soft until autofocus catches up.