(Capcom's primary storyline)
P-ε (P-Epsilon) or known by its development code UMB No.21 is an Umbrella-created toxic gas. It was designed to stop out-of-control B.O.W.s who have broken out. However, when in contact with Plant 43, it causes them to mutate into a more dangerous form, where they create a poison ability. The stated explanation is that some species are able to extract toxins from the gas and use these toxins against threats.
In the 2019 remake of Resident Evil 2, P-ε is specifically an herbicide. It must be synthesized and deployed to damage Plant 43 to open up an area of the greenhouse . Synthesis involves cooling the subtance to cryogenic temperatures.
Earlier in the franchise development, P-ε was an anti-"t" gas, as it was Lore at the time that only certain enemies were infected with the virus. The Ivies, not infected with "Epsilon", are thus not destroyed by the gas.