If your classmate spreads a false rumor that you cheated on the math test, that's slander. Slander is the act of saying an untrue, negative statement about someone.
In law, the word slander is contrasted with libel, which is the act of making a false written statement about someone. The noun slander is from Old French esclandre, escandle, or "scandal," from Late Latin scandalum "stumbling block, offense."