In HVAC, optimum stop is a control strategy that turns off the cooling compressor (or heating equipment) but keeps ventilating the building during the time period when occupants are getting ready to leave. Depending on the building, this can be as much as 1.5 hours to 15 minutes before the building becomes unoccupied. The building temperature drifts slightly during this time period. Optimum stop can be used in a building that uses optimum start, but the two strategies are completely separate and not dependent on each other (as is a common misconception)
Example:
A simple example of optimum stop is turning the air conditioner off on your car for the last few minutes of your drive (you will notice the vented air remains cold for short period of time). In a building, it works essentially the same, but since the building has so much more mass, the time it takes to change temperatures is much greater than a car.