Incorrect spelling, explanation: rehersal is the wrong form, as the word comes from the word hear – rehear: hear again. We spell hear with the vowel a so it remains in rehearsal, and rehearsal is the only correct form.

Correct spelling, explanation: this word is spelled with ea because of its origin. Rehearsal derives from the word rehear as hear one more time. We spell hear with ea, therefore the correct form is rehearsal, not rehersal. The mistake may come from the spelling.

Definition of rehearsal:
noun, a session to practice and prepare for something; usually for a public performance
This is our fifth rehearsal and you still forget your lines, this is unacceptable.
I feel that I am always better at the rehearsals than the actual play!