A rumor is a piece of information whose truth or source is unknown. Rumors are known for spreading between people quickly and easily since they are easy to tell, but hard to prove!
For example, if someone at school tells you that your teacher has a pet tiger named Fluffy, that would be a rumor. Rumors often start out with information that is true ("Our teacher has a pet") which then get exaggerated and change into something false ("Our teacher has a pet tiger"). A lot of the time, rumors contain hurtful information.
Since it's not a good idea to say mean things and since rumors are often not even true, it's not a good idea to spread a rumor. If you hear something that sounds like a rumor, you don't need to tell it to other people unless the information contains something hurtful or dangerous that a grown-up needs to know about.
(by Mya Kagan)