Acid is a light green liquid substance similar to water introduced in level 4 of Plazma Burst 2. This is a dangerous substance, it is corrosive and drains the health of anything in it. The rate of how much HP is lost depends on how strong the acid is. You can roll your cursor over it to see how much damage it deals per second and, based on that number, how many seconds it takes to die.
To make acid in a custom map, you only have to place a water area and set its damage to a positive value. Note that the value you set is damage in 1/30' of a second, meaning that the actual damage per second will be 30 times higher than the value you input. For example, if you input 10, acid's damage will be 300 health points in a second.