Sin is the act of breaking a religious or moral law, especially in Christianity and other faiths. When someone commits a sin, they do something their religion teaches is wrong, like lying, stealing, or hurting others. Different religions have different ideas about what counts as sin, but most agree that sins are actions that damage our relationships with God, other people, or ourselves.

The word can also be used more casually. Someone might jokingly say that eating a third piece of cake is “such a sin,” meaning it feels indulgent or wrong even though it's not actually a religious matter. In mathematics, sin is short for sine, a function used in trigonometry, which has nothing to do with the religious meaning.

As a verb, to sin means to commit a sin.

The concept of sin reminds believers to think about their choices and try to do better, while also recognizing that nobody's perfect.