The ability to generate and/or manipulate toxic substances.
Wikipedia defines poison as thus:
In the context of biology, poisons are substances that can cause disturbances to organisms,usually by chemical reaction or other activity on the molecular scale, when a sufficient quantity is absorbed by an organism.
Venom or Toxin is a subtype of poison that is created by living organism, and would be considered a real life example of poison manipulation. in fiction, acid is often incorrectly labeled as poison and or poison is shown to act like stereotypical acid, and therefore this page will also include acid and its manipulation as well.
Could be considered a form of Elemental Manipulation.
Also known variously as Miasma, corruption and blight. Radiation manipulation may or may not fall under this category depending on the series.