Warming up. Getting ready. Preparing to perform.
The main task in preparing to do a performance is, I think, a mental one. You are preparing to submit to the task of performing. The fear, the boredom, the elation, the physical work, the concentration, the agility, great success, and failure.
Rehearsal is about mastering the skills necessary for the performance. Rehearsal is a physical process. It is about getting in control. Of the character, the lines, the plot, the physical tasks. Performing is about letting go. Maybe even losing control. Giving it up to the audience.
It's really important not to confuse the two. Performing is about energy and focus, rehearsing is about analysis and repetition.
Performing requires a lot of concentration, a lot of energy, and the willingness to follow wherever the show leads. You have to work yourself up for it over and over again. To do this you might run through bits of the show and try to sharpen them up. This inevitably throws up questions - how do we do this? What is the best way to do that? Which drags you into rehearsal mode. So you might switch into rehearsal mode, which is fine, but it isn't preparing you to perform. Even if you continue to rehearse after you open a show you must perform like you have finished rehearsing.
To prepare to perform you must set out to be brilliant every time, and you must prepare for failure.
This is a mental preparation.