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 models reach 70 hours and beyond.
A mechanical watch runs on energy stored in its mainspring; when that energy runs out, the watch stops. The power reserve tells you how long the spring can keep the watch going. A typical 40 hours means a watch worn every weekday runs fine, but take it off Friday evening and it may have stopped by Monday. Longer-reserve movements of 60 to 80 hours solve that. Some watches have an indicator showing how much reserve is left.