What term describes a substance that is poisonous to living organisms?

The term that accurately describes a substance that is poisonous to living organisms is "toxicant." A toxicant specifically refers to any harmful chemical that can cause adverse health effects in living organisms, which can include a wide range of substances, both natural and synthetic.

While "toxin" is closely related, it typically refers specifically to poisonous substances that are produced by living organisms, such as bacteria, plants, or animals. "Toxic" is an adjective used to describe a substance that is poisonous, rather than the substance itself. "Toxicity," on the other hand, is a measure of the harmful effects of a substance, not the substance itself. Therefore, the best choice to denote a substance that exhibits poisonous properties is "toxicant."