The transverse arm, north or south, of a cross-shaped church.
Slype
Roughly, Gothic architecture, mainly in churches and cathedrals, has arches that come to a bit of a point at the top, steeply-pitched stone roofs (previous architecture had flatter wood roofs that would burn and collapse into the church), and had interior outer aisles with clerestory windows up high, and also vaulted ceilings whose crossings feature groined 'joints'.
Choir or transept.
the Crossing
Slype
Transept The ground plan of many churches forms the shape of a cross. The two 'arms' of the cross are the transepts.
The Slype is a covered passage, especially one from the transept of a cathedral to the chapter house. It is famous that a Slype is a covered passageway in a cathedral or church that connects the transept to the chapterhouse.
Slype - found in Chambers Dictionary
This is called the 'transept' and its purpose is to provide separation between the Sanctuary and the Nave. Sometimes the transept will include an area for the choir (although a loft is more common) and/or statuary, or side altars.
Poets corner
If you mean the side arms of a church, it is called "the transept". Each of the two may contain a chapel.
Sunday Express: transept
A transept actor refers to a performer in a specific genre of theater that often involves religious or moral themes, particularly in medieval mystery plays. In this context, "transept" typically relates to the architectural feature of a church, signifying the area that intersects with the main body, where dramatic performances may have historically taken place. These actors would typically portray biblical figures or moral allegories, engaging the audience in narratives that reflect spiritual teachings.
It did not 'burn down' however a fire in 1984 destroyed the roof in the south transept.