A pentamirror is a collection of mirrors typically incorporated within the viewfinder of a single-lens reflex camera which is shaped such that when the photographer moves their camera up and down or left and right the image within the viewfinder moves in the expected direction.
Pentamirrors are cheaper to make than pentaprisms and tend to be found in lower-end DSLR cameras.
For a discussion of the advantages and disadvantages for pentamirrors, check out this article.