1A stone forming part of or constituting an altar; especially the slab forming the flat top or table of an altar.
2Specifically. In the Roman Catholic Church: a consecrated portable slab of marble or stone, typically containing a relic, which may be placed on or inserted into a larger altar or table, or can be used by a priest when Mass is said somewhere other than a church.
altar stone/ˈɔːltə stəʊn/ /ˈɒltə stəʊn/
Late Middle English; earliest use found in William of Shoreham (fl. c1320), poet. From altar + stone.