Damage per shot (often shortened to DPS) is used officially in every game of the main Plants vs. Zombies series to express the damage plants deal and the amount of health zombies have. Below is the explanation of such.
Some projectiles from plants can do more than just 20 DPS, and can do up to about 80 DPS, such as Cabbage-pult, Melon-pult, and peas ignited by a Torchwood. The projectile with the most damage output is Citron's charged plasma ball created during its Plant Food effect, dealing about 3600 or less DPS.
Instant kills deal a heavy amount of damage, all dealing about 1800 DPS except for Primal Potato Mine, which deals 2400 DPS. However, this may be disproved in ZomBotany 2 where a full-health Tall-nut Zombie can be exploded by a corn cob, Cherry Bomb, Jalapeno or Doom-shroom, even though it deals more damage than what instant kills usually deal with other zombies, so this may be a glitch. However, Potato Mines or Squashes cannot kill a full-health Tall-nut Zombie, despite having the same damage as the plants that cause explosions above. Chompers deal 40 DPS (200 in the sequel) to any zombie it cannot devour.
1 Normal Damage Shot (NDS) is equal to 20 DPS in most situations.