Perjury is the crime of lying under oath. When someone testifies in court, they first promise to tell the truth, sometimes by placing their hand on a Bible or by making another formal promise. If they deliberately lie after making that promise, they commit perjury.

The word comes from legal settings where truth is essential. Courts depend on witnesses telling the truth so judges and juries can make fair decisions. If a witness saw a car accident and later testifies in court that the light was green when it was actually red, that's perjury. If someone claims under oath that they were home all evening when they were actually somewhere else, that's perjury too.

Perjury is serious because lying in court can send innocent people to jail or let guilty people go free. It undermines the entire justice system. Just being wrong or misremembering isn't perjury: the person must know they're lying and do it on purpose.

The consequences can be severe. Someone convicted of perjury can face prison time and fines. Beyond legal punishment, being caught committing perjury destroys a person's credibility. Once a person has lied under oath, people have good reason not to trust their word again.