Z is for Zoom
A zoom lens is a lens with an adjustable focal length, allowing the photographer the flexibility to get more or less environment in the photo with their lens, not with their feet.
The zoom lens (with the exception of the most expensive lenses) trade off optical quality for convenience, a sacrifice many photographers are prepared to accept. Optical quality is hardly poor on good zoom lenses, but the difference is clear when compared to a comparable prime lens at the same focal distance. Optimum optical quality is usually found 2 or 3 stops higher than the widest aperture the lens can support.