A sanctuary, in its original meaning, is a sacred place, such as a shrine, protected by ecclesiastical immunity. By the use of such places as a haven, by extension the term has come to be used for any place of safety. This secondary use can be categorized into human sanctuary, a safe place for people, such as a political sanctuary; and non-human sanctuary, such as an animal or plant sanctuary.

Sanctuary is a word derived from the Latin sanctuarium, which is, like most words ending in -arium, a container for keeping something in—in this case holy things or perhaps cherished people (sanctae/sancti). The meaning was extended to places of holiness or safety. Its origin is the principle of independence and immunity of religious orders from "temporal" powers.

In many religious buildings sanctuary has a specific meaning, covering part of the interior.

In many Western Christian traditions including Catholic, Lutheran, Methodist, and Anglican churches, the area around the altar is called the sanctuary; it is also considered holy because of the belief in the physical presence of God in the Eucharist, both during the Mass and in the church tabernacle at other times.

In many churches the architectural term chancel covers the same area as the sanctuary, and either term may be used. In some Protestant churches, the term sanctuary denotes the entire worship space while the term chancel refers only to the area around the communion table.

In many Western traditions, altar rails and sometimes steps would demarcate the sanctuary or chancel from the rest of the building. In the Eastern Orthodox Church, Eastern Catholic Churches of Syro-Malabar Church, Byzantine rite and Coptic Orthodox Churches, the sanctuary is separated from the nave (where worshippers pray) by an iconostasis, literally a wall of icons, with three doors in it. In other Oriental Orthodox traditions, a sanctuary curtain is used.

The terminology that applies the word sanctuary to the area around the altar does not apply to Christian churches alone: King Solomon's temple, built in about 950 BC, had a sanctuary ("Holy of Holies") where the Ark of the Covenant was, and the term applies to the corresponding part of any house of worship. In most modern synagogues, the main room for prayer is known as the sanctuary, to contrast it with smaller rooms dedicated to various other services and functions.

In synagogues there is a raised bimah in the sanctuary, from which services are conducted, which is where the ark holding the Torah may reside; some synagogues, however, have a separate bimah and ark-platform.