An equatorial mount is a telescope mount including an axis parallel to Earth's so by rotation around that axis, the telescope can maintain its target in view.
The alternative mount is the altazimuth mount which requires coordinated rotation around two axes to keep its target in view. The equatorial mount has been considered an advantage in that if the one axis is designed for very smooth movement, the telescope can keep the image very stably positioned, which is very helpful for long exposures.
Equatorial mounts require extra mechanical hardware that makes them cumbersome and expensive for the largest telescopes. Over time, computer control to work around the need for an equatorial mount has become more practical, effectively reducing the maximum aperture at which an equatorial mount is likely to be desirable.
Some major telescopes with equatorial mounts: