Real Dictionary

dress rehearsal
countable noun: The dress rehearsal of a play, opera, or show is the final rehearsal before it is performed, in which the performers wear their costumes and the lights and scenery are all used as they will be in the performance.
countable noun: You can describe an event as a dress rehearsal for a later, more important event when it indicates how the later event will be.

dress rehearsal in American English
a final rehearsal, as of a play or ceremony, performed exactly as it is to take place
noun: a rehearsal of a play or other performance in costume and with scenery, properties, and lights arranged and operated as for a performance: often the final rehearsal

dress rehearsal in British English
noun: the last complete rehearsal of a play or other work, using costumes, scenery, lighting, etc, as for the first night