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.
The name for the aisle that crosses the nave is called the transept. It typically intersects the nave at a right angle, creating a cross-shaped floor plan in churches with a cruciform layout.
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
Transept The ground plan of many churches forms the shape of a cross. The two 'arms' of the cross are the transepts.
The parts inside a church are: aisle, almonry, ambulatory, chancel, chapel, chantry chapel, Lady chapel, chapterhouse, choir, cloister, crossing, crypt, presbytery, sacristy, vestry, sanctuary, nave, slype, and transept. hope this helps !
If there is only one room, then it is just called "the chapel", if it is divided, the sanctuary is where the altar is, and the nave would be outside the sanctuary where there would be pews, etc. If there is an anteroom, it is called a narthex.
The Nave is the middle part of the Church. This is where the congregation sits during the service, and is flanked by aisles on each side.
A typical church building is usually in the shape of a cross when viewed from above, with a long central nave and shorter arms forming the transept. This design is symbolic of the Christian faith and serves as a reminder of the crucifixion of Jesus Christ.
Nave
Nave
It isn't called anything. It is just called part of the nave.