Pentamirror
A lighter, cheaper alternative to the pentaprism using a hollow arrangement of mirrors to perform the same image correction.
In depth
Each mirror surface reflects perhaps 90 to 96 per cent of the light rather than the near-total internal reflection of solid glass, so a pentamirror finder is noticeably dimmer and often shows less than full frame coverage. The saving in weight and cost is substantial, which is why the design defines the entry-level DSLR.
Examples in practice
Entry-level DSLRs typically pair a pentamirror with 95 per cent viewfinder coverage, so a little more appears in the file than was seen.