Venom is natural poison secreted by animals. Venomous animals on Earth include species of snakes, arachnids, and insects.
A mugato's fangs contain venom that attacks its prey's nervous system, killing it after a few hours. Mahko root is the only known antidote. (TOS: "A Private Little War")
Lieutenant Worf, suffering from Barclay's Protomorphosis Syndrome in 2370, developed a venom sac beneath his jawline and began spitting the acidic compound throughout the USS Enterprise-D. When Dr. Crusher discovered the sac on his neck, Worf spit the venom in her face, severely disfiguring her. He later attacked Captain Picard in the same fashion. (TNG: "Genesis")
The venom of Regalian fleaspiders can improve a humanoid's blood circulation. (DS9: "The Ship")