The Pentamirror is kind of a "fake pentaprism". A pentaprism is an optical glass element with 5 planes defining the optical path in a pentaprism viewfinder of a common single lens reflex camera. It must be ground very precisely. Thus it has become cheaper for the camera makers to replace that expensive glass element by a mirror construction called "pentamirror" which fits in a normal "pentaprism" housing. The devices are lighter but seem to be a little darker than pentaprisms. Sometimes new cameras with pentaprism have a design which emphasizes the pentaprism form, suggesting to represent classical optics (example: Sony Alpha DSLR-A900).