Illustrated Architecture Dictionary
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A religious sanctuary may be a sacred place (such as a church, temple, synagogue or mosque), or a consecrated area of a church or temple around its tabernacle or altar.
In a cruciform church, the immediate area around the principal altar, which is enclosed by a communion rail, setting it off from the nave
The words chancel and sanctuary are often synonyms. Protestant churches tend to use "chancel," whereas Catholic churches tend to use "sanctuary."
Examples from Buffalo architecture: