(~s)
1.
The ~ 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.
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2.
You can describe an event as a ~ for a later, more important event when it indicates how the later event will be.
These elections, you could almost say, are a ~ for the real elections.
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