A poison is a chemical that can cause harm and possibly death to an organism. This tag should be used for horticultural questions about poisons of all kinds, including both plant and animal poisons.
A poison might be deliberately used to control a particular pest, but care should be taken so that they do not poison other desirable plants and animals (including humans). Similarly, some plants produce their own poisons (e.g. nightshades) and these would be on-topic for this tag.
Finally, most poisons are relatively benign in very small quantities; but a few are actually considered essential nutrients. For example, copper is an essential, if minor, plant nutrient; but it is often employed as an effective tree poison.