Power Reserve
Power reserve is how long a fully wound mechanical watch keeps running off the wrist. On most automatics it is around 40 hours; some movements reach 70 hours and more.
A mechanical watch runs on the energy stored in its mainspring; once that energy is spent, the watch stops. The power reserve tells you how long the spring can keep it going. A typical 40 hours means a watch worn every weekday is fine, but take it off on Friday evening and it may well have stopped by Monday. Longer-reserve movements of 60 to 80 hours solve this. Some watches carry an indicator showing how much reserve remains.