Zoom lenses, also called parfocal lenses, are a special type of varifocal optics in which the working distance is kept constant when changing focal length (i.e. the focus is maintained throughout the process).
Zoom lenses are often described by the ratio of their longest to shortest focal lengths or by the ration between their maximum to minimum magnification.
In this category, we can also find macro zoom (e.g. Opto Engineering® MCZR and MZMT) and telecentric zoom lenses (Opto Engineering® TCZRS).