Energy cell

The cell is the unit of ammo used by the plasma rifle and BFG 9000 weapons. The number of cells you are carrying and your current maximum are shown on the bottom line of the ammo section at the right of the status bar. One quick tap of the fire key while the plasma rifle is selected consumes one cell.

Perhaps confusingly, the printed booklets that come with the games show "Cell" as an ammo pickup. This pickup actually contains 20 cells (40 at I'm Too Young To Die skill level), and the game calls it an energy cell. Other sources call this pickup a cell charge.