Pentaprism
A solid five-sided glass prism above an SLR's focusing screen that corrects the image left-to-right and presents it at eye level.
In depth
The mirror alone produces an image that is upright but laterally reversed; the pentaprism folds the light path three times to correct it. Because it uses total internal reflection through solid glass, it is bright and contrasty but heavy and expensive, which is why it distinguishes professional bodies from those using a hollow pentamirror.
Examples in practice
A pentaprism finder stays usably bright with a slow f/5.6 zoom fitted, where a pentamirror would look dim and grey.