The usual term is constant-aperture zoom lens, or more precisely constant maximum aperture zoom lens.

That means the lens can maintain its widest aperture across the full zoom range — for example, f/2.8 at 35mm and also f/2.8 at 100mm. By contrast, a variable-aperture zoom might be labeled f/3.5–5.6, meaning the widest available aperture gets smaller as you zoom in.

Including the word maximum can be helpful, because fixed aperture can mean something different: a rare lens design that only has one aperture setting and cannot be stopped down. In everyday use, though, people often just say constant-aperture zoom and will be understood.