The Celestron 90-Degree Mirror Diagonal (2") is a star diagonal with StarBright XLT coating, for use with Schmidt-Cassegrain telescopes with the 2"-18 threaded device.
Star diagonal is the term commonly associated with 90° mirror diagonals such as this, which let you view objects that are at a 90° angle from the direction the telescope is pointing. This allows quite comfortable viewing when the telescope is at or near the zenith (directly overhead), and is fairly comfortable when observing terrestrial imagery as well-albeit a bit unorthodox at first.
Celestron's erect image diagonals are Amici prism designs, meaning that images are upright and laterally correct. This design, however, instead renders images which are laterally reversed but are brighter than erect image diagonals. This diagonal is used with 2" focusers and can be used with either 2.00" or 1.25" eyepieces, thanks to the included adapter which reduces the eyepiece barrel diameter to accept the smaller size.