Ad Orientem means "toward the East"

source: New Liturgical Movement
by Jeffrey Tucker
edited by Soutenus

What does it mean for us to "look east?"
The early Church attached great importance to facing towards the dawning light in its prayer. Early churches were built so that, when the assembly gathered for prayer, they faced the East. When Mass was celebrated, priest and people faced not each other, but together faced the altar, toward the East.
St. Clement of Alexandria (150 – 216 AD) explained:

... And since the dawn is an image of the day of birth, and from that point the light which has shone forth at first from the darkness increases... In correspondence with the manner of the sun’s rising, prayers are made looking towards the sunrise in the east. (Stromata Book IV, ch. 7)

Even when, as the Church grew, it was no longer possible to build every church so as to have the altar facing eastward, the custom remained of having priest and people together face the altar during the Eucharistic liturgy, facing the Daystar who came to be with His people on that altar.

This posture of priest and people facing the altar is known as ad orientem, which is the Latin for “toward the East”. Most Catholics who are aware of this posture would probably associate it with the Extraordinary Form of the Mass, that is, the Mass as it was celebrated before the liturgical reforms of the 1970’s. Indeed, one of the liturgical changes most associated with Vatican II is that of turning the priest around so that he faced the people.

Many Catholics would probably imagine that this change was mandated by Vatican II, and that the former posture of ad orientem had been abolished. But this impression, widespread though it is, is incorrect. In point of fact, no document of Vatican II and nothing in the rubrics of the modern Roman Rite either requires the priest to celebrate Mass facing the people or abolishes celebrating Mass ad orientem.

photo credit: Homesick No More