In the strictest sense, a Cassegrain telescope is a reflecting telescope that uses two curved mirrors of particular shapes to form the image: The primary mirror is a concave paraboloid, just as in a Newtonian. The secondary mirror is a much smaller convex hyperboloid, positioned a little way toward the primary from the primary's focal plane. The secondary reflects the converging beam of light back toward the primary, which typically has a hole in the center, so that the focal plane of the combined mirrors is easily accessible behind the primary.
More loosely, the term "Cassegrain", or "Cassegrain configuration", or "Cassegrain system", has come to be applied to any telescope in which a large primary mirror and a small convex secondary mirror are among the elements used to form the image. Common Cassegrain systems include the Schmidt-Cassegrain, in which a corrector plate which appears flat to the unaided eye is mounted just skyward of the secondary mirror, and several varieties of Maksutov-Cassegrain, in which a correcting lens that resembles a thick watch glass is similarly mounted. In some Maksutov-Cassegrains, the secondary mirror is an aluminized spot on the correcting lens.
There are many other Cassegrain systems besides the ones just mentioned. In some, the only difference from the classical Cassegrain is that the two mirrors are not the traditional paraboloid/hyperboloid combination.

This definition is in context to Telescope. See more contextual defintions for Cassegrain.