Energy

Your power is the amount of magical energy you have:

Dlvl:3 $:43  HP:68(68) Pw:13(18) AC:-3  Xp:8/1295 T:2358

The above status line shows a character whose current power is 13 and maximum power is 18.

Power is consumed by casting spells, and regained slowly while waiting, quickly by quaffing a potion of gain energy, and fully by reading a scroll of charging while confused.