A rehearsal was a practice session for an upcoming speech, play, or musical performance.
After travelling back from 2368 to 1893, Captain Picard and his officers claimed to do rehearsals for A Midsummer Night's Dream in Mrs. Carmichael's flat. (TNG: "Time's Arrow, Part II")
In 2259, James T. Kirk, bewildered by the crew of USS Enterprise breaking out into song and dance, admitted he'd assumed (incorrectly) that it was a previously rehearsed performance, not spontaneous. (SNW: "Subspace Rhapsody")
In 2367, Keiko Ishikawa cancelled the planned wedding rehearsal with Data, since she had decided not to marry Miles O'Brien, though she later changed her mind. (TNG: "Data's Day")
Later that year, Reginald Barclay commented to Deanna Troi that he had made progress in his performance of Hercule-Savinien De Cyrano de Bergerac only after countless rehearsals. (TNG: "The Nth Degree")
Also that year, Data planned to give more attention to the rhythmic patterns of a musical piece at the next rehearsal involving him and other performers, including Jenna D'Sora and Keiko. (TNG: "In Theory")
In 2369, Benjamin Sisko sharply told Julian Bashir the reception for welcoming the Wadi to Deep Space 9 wasn't just a dress rehearsal, but "the real thing" when Bashir didn't wear his dress uniform. (DS9: "Move Along Home")
The same year, a rehearsal of Something for Breakfast took place where Jean-Luc Picard was supposed to be the butler. (TNG: "A Fistful of Datas")
Later that year, William T. Riker rehearsed for his part in Frame of Mind. (TNG: "Frame of Mind")
In 2371, Ulani Belor confided that she and Gilora Rejal had spent most of the time traveling from Cardassia Prime to DS9 rehearsing what to say upon arrival. (DS9: "Destiny")
When the USS Voyager traversed the Void in 2375, The Doctor planned a rehearsal of a duet from Don Carlos on the holodeck. (VOY: "Night")
Later that year, Data rehearsed a production of HMS Pinafore before his mission to the Ba'ku planet. (Star Trek: Insurrection)
For the play written by Kelis in 2376, several rehearsals took place even though there was no written ending until the actual performance. (VOY: "Muse")