To rat means to betray someone by telling on them, especially to get them in trouble or to help yourself. When someone rats on their friend for breaking a rule, they're revealing a secret to an authority figure like a teacher or parent. The word carries a negative feeling because it suggests disloyalty or betrayal, particularly when done to save yourself from consequences.

People sometimes argue about whether telling an adult is ratting or just being responsible. There's a difference between ratting and reporting something genuinely dangerous. If your friend is doing something that could hurt someone, telling a teacher is protecting people, not ratting. But if you tell on your friend just because you're angry at them, or to make yourself look better, that's when people might call you a rat.

In old crime stories, a rat was someone who betrayed their partners by giving information to the police.

As a noun, a rat is also a rodent: a small mammal with a long tail, sharp teeth, and a pointed nose. Rats are intelligent, adaptable creatures found almost everywhere humans live.