Versus populum
Versus populum (Latin for "towards the people") is the liturgical stance of a priest who, while celebrating Mass, faces the people from the other side of the altar. The opposite stance, that of a priest facing in the same direction as the people, is today called ad orientem (literally, "towards the east" − even if the priest is really facing in some other direction) or ad apsidem ("towards the apse" − even if the altar is unrelated to the apse of the church or even if the church or chapel has no apse). In the early history of Christianity it was considered the norm to pray facing the geographical east. From the middle of the 17th century, almost all new Roman Rite altars were built against a wall or backed by a reredos, with a tabernacle placed on the main altar or inserted into the reredos. This meant that the priest turned to the people, putting his back to the altar, for a few short moments at Mass. However, the Tridentine Missal is not celebrated versus populum since the Ritus Servandus gives corresponding instructions for the priest when performing actions that require him to face the people. In the Ritus Servandus, the rubrics say "with his hands joined before his breast, and with his eyes downcast, he turns toward the people from left to right." This would otherwise not make sense in the context of versus populum since versus populum assumes that he is already facing the people. (Wikipedia).
