Structure forming the transverse part of a cruciform church; crosses the nave at right angles WordNet 3.0 & Open English WordNet

A transept is a transverse part of any building, which lies across the main body of the building. Wikipedia

Architectural term Wikidata

The transversal part of a church, which crosses at right angles to the greatest length, and between the nave and choir. In the basilicas, this had often no projection at its two ends. In Gothic churches these project greatly, and should be called the arms of the transept. It is common, however, to speak of the arms themselves as the transepts. Wiktionary