A small flat stone (usually square, approximately 6" x 6" to 12" x 12") consecrated by the proper high church official, and on which the Sacrifice of the Mass may be said. It may be placed and used on an altar not yet consecrated and thus becomes the true altar. (An optional piece not found in all churches.)

the stone in the middle of the inner horseshoe, so named because when explorers found the stone, it was on the ground like an altar

The Altar stone is a central megalith at Stonehenge in England, dating to Stonehenge phase 3i, around 2600 BC. It is made of green micaceous sandstone and originated from a quarry in Wales. It weighs six-tons and would have been twice the height of the bluestones at the site.