It’s any substance that is toxic — meaning it can harm health
Poisonous (adjective, “POY-suh-nuss”)
In most uses, this word refers to something that can harm or kill an organism. But in biology, only some organisms that make a toxic substance are considered truly poisonous. To get that name, they must secrete the chemical passively. Then it just stays on or inside the organism until somebody — or something — eats it. Plants can be poisonous. So can animals that secrete toxins through their skin. But if an animal has to bite or sting to deliver its toxic chemical, scientists call it something different: venomous.
In a sentence
While a poison arrow frog is poisonous, and a rattlesnake is venomous, both are dangerous.
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poison A substance that causes sickness or death to an organism.
poisonous (In biology) An organism that secretes a toxic substance passively. Plants can be poisonous, as can animals that secrete toxins through their skin.
toxic Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.
toxin A poison produced by living organisms, such as germs, bees, spiders, poison ivy and snakes.
venom A poisonous secretion of an animal, such as a snake, spider or scorpion, usually transmitted by a bite or sting.