TRANSEPT

The Transept is a  rectangular space inserted between the apse and nave in the early Christian basilica. It sprang from the need of procuring sufficient space for the increased number of clergy and for the proper celebration of the service. The bay at which the transept intersects the main body of the church is called the crossing. The transept itself is sometimes simply called the cross.The length of the rectangle either equals the entire breadth of the nave, , or it exceeds this breadth more or less, so that the transept extends beyond the walls of the nave. The transept generally terminates towards the north and south in a straight line.

Picture taken from: http://www.pitt.edu/~medart/menuglossary/transept.htm

Acknowledgement: http://www.newadvent.org/cathen/15018a.htm
                              http://www.britannica.com/EBchecked/topic/602515/transept