Poison/ˈpɔɪzn/ Definition 1 of 4Pro Tip 1/2Literal Harm Used to describe substances with actual, physical harmful effects on living beings. The factory released poison into the river, killing the fish. Pro Tip 2/2Varying Lethality Poisons can range from causing mild illness to immediate death, depending on type and amount. The poison in the berries can cause nausea if eaten.