What is the meaning of Ad_orientem?

Facing liturgical east, that is, in the same direction as (and thus, away from) the congregation.

Describing worship facing liturgical east.

Ellipsis of ad orientem worship, posture, etc.

oriented in the same direction as fellow communicants during mass (incidentally away from the people)

ad orientem (facing liturgical east, that is, in the same direction as and away from the congregation)

Source: wiktionary.org

See also