A poison is a substance, either organic or chemical, which inflicts harm upon a living being. In the context of the PPC, those beings are usually Mary Sues, whom can only be called 'living' by a very, very technical definition.
There are a wide variety of poisons available in the multiverse, but during missions, assassins are limited to substances that exist within the assigned continuum so as not to leave behind anything uncanonical in the corpse.
Substances which are harmless in one continuum may be deadly in others. When crossovers are involved, agents may need to an Anachronistic Biochemical Substance Eliminator.
An extremely good book for choosing just the right poison for an assasination is Deadly Doses: A Writer's Guide To Poisons, which Acacia read in her spare time. Agent Sedri owns a copy.