Perjury is the act of deliberately lying under oath. A defendant in a murder trial commits perjury when he swears he never met the murder victim before, even though he had a two-year relationship with her.
Although the word perjury contains jury, members of the jury aren't likely to commit perjury, because they're not under oath and haven't sworn to be truthful. A witness in a trial can commit perjury by willfully lying about facts related to the case. Perjury isn't just a little white lie. It's a crime that can land the person who commits it in jail.