This term comes from the Latin "Septum", meaning an enclosure, and refers to the space that separates the nave from the chancel.
In our church the transept is the carpeted aisle that runs perpendicular to the centre aisle, beginning at the Baptistry (where the baptismal font is) and ending at the double doors which lead to the south-east entrance to the church.
In our church the length of the transept is less than the nave.
Interestingly, in many churches whose design is cruciform (i.e., in the shape of a cross) the transept is longer than the nave, thus creating the perception that the interior of the church is one big cross whose arms are transept.
However, even though our transept is not of cruciform length, one gets the impression when looking down our carpeted aisle, that we worship at the foot, in the arms, and at the head of a great red cross; what a fitting image for us, who proclaim that the way of the cross is the way to eternal life.