1.NounA rehearsal prior to a public spectacle, where all the performers are in costume, in order to test all aspects of the production by closely reproducing the actual conditions of a performance.
"We've got three dress rehearsals before the opening night to prepare ourselves for any potential technical issues."
2.NounThe final rehearsal before a performance, whether or not costumes or performance dress are worn.
3.NounAny complex activity that serves to prepare for a more important iteration of it later.
"Although the train derailment ended up not being very serious, the emergency services' response served as a dress rehearsal in case of a much worse disaster."