A dress rehearsal is a full-scale rehearsal or practice of a theater or musical performance. This practice is intended to perfect the performance before the public sees it, making sure every detail of the show meets expectations. The cast members wear their costumes and accessories during the dress rehearsal. They may even have the practice in front of the backdrop and using the props designed for the upcoming theater performance. For musical performances, however, this is not the case, as a dress rehearsal is simply the final rehearsal for musicians and doesn't require donning the attire intended for the show.
In theater, a number of rehearsals happen long before the dress rehearsal. These practices are often called run-throughs. A play run-through isn't done in costume and may not cover every technical aspect of the upcoming performance. Instead, these practices are intended to help the cast learn the dialogue and understand how each person will be positioned during the public performance.
Sometimes, a theater dress rehearsal is just one final practice, but it may actually refer to a series of practices. In this type of rehearsal, the cast practices the entire performance, going through all the motions it will for the public performance. It will even stop for intermissions, just as it will for the actual performance. In an open dress rehearsal, certain people may be allowed to attend and watch the practice. This privilege is typically extended to family and friends of the cast as well as theater reviewers.
Another type of rehearsal, called the cue to cue, is used to perfect technical aspects of a play or show. In this type of rehearsal, the stage manager and technicians have the cast rehearse dialogue and movements that can be used as cues for technical changes, such as lighting, sound, and any special effects. This allows them to make sure everything works as it should and iron out any glitches. Costumes are generally not necessary for this type of rehearsal. However, they may be donned in some cases to see how they'll look against the stage lighting.
Sometimes, there is a preview performance after the dress rehearsal. Technically, this is not a practice, as it takes place in front of a full audience. However, it is not uncommon for the cast to stop and start over from a previous point in the performance if problems or glitches arrive. Typically, this doesn't take place during a full-scale performance.