Poison is a substance that can harm or even kill you. Arsenic and cyanide are both types of poison—so stay away from them! Mean people can also be poison, so stay away from them too.
Poison refers to the harmful substance itself, or the act of giving it to someone. Poison often shows up in murder mysteries because it’s a sneaky way to get rid of someone. We also use poison metaphorically for things that tend to ruin something. A really obnoxious friend who always insists that he knows more than everyone else might poison the atmosphere at a dinner party. Jealousy can poison a good relationship.