A rattler is a rattlesnake, a venomous snake found throughout the Americas that has a distinctive rattle at the end of its tail. When threatened, a rattlesnake shakes this rattle rapidly, creating a buzzing sound that warns intruders to stay away. The rattle is made of hollow, interlocking segments of hardened skin that knock against each other when vibrated.

Rattlesnakes use their warning rattle as a defense mechanism because they would rather scare you off than waste their venom biting you. If you're hiking in rattlesnake country and hear that distinctive buzzing sound, stay still and look carefully around you to spot the snake, then move away slowly. Rattlesnakes won't chase you. They just want you to leave them alone.

The word rattler comes from the sound these snakes make, and people sometimes use it as casual shorthand for rattlesnakes. A park ranger might warn hikers, “Watch out for rattlers on this trail,” or a desert guide might say, “I spotted a big rattler sunning itself on that rock this morning.”

Some people also call old, noisy cars or trains rattlers because of the shaking, clattering sounds they make, though this usage is much less common.