Another name for a church niche could be:
* Apse (Technically, an apse is the semicircular or polygonal recess, often at the east end, that *contains* the altar, but the niche might be part of the apse or a separate feature within it.)
* Alcove
* Recess
* Chapel (if it's a larger, dedicated space within the church)
* Bay (if it's a structural element forming a niche-like space)
* Tabernacle (specifically if it houses the Eucharist)
* Shrine (if it contains a relic or an image of a saint)
* Sanctuary (although this usually refers to the holiest part of the church, surrounding the altar, a niche may be within the sanctuary)
* Ambry (specifically if the niche is used to house holy oils)
* Aumbry (alternative spelling of ambry)
* Piscina (if the niche contains a basin used for washing communion vessels)
* Sedilia (if the niche contains seating for the clergy)
The best alternative depends on the specific function and location of the niche within the church.