I define it by whether something is independently verifiable.

I am told that there are 8* planets in our solar system, and where they are located. If I wanted to, I could buy a big telescope, point it at the sky and find all 8.

I am told that it is possible to boil water through nuclear fission. If I had the means, I could take a number of resources, spend decades researching nuclear physics, build my own test reactor, and verify that this is possible.

I am told that the earth is flat. I could get a pilots license, buy a plane, and fly to Antarctica to see the ice wall. I would find that there is no ice wall, just a number of scientists who are very passionate about ice samples. Therefore, it is not independently verifyable.

I don't have the money to verify all of these claims, but they are all claims that have been verified by hundreds, if not thousands of independent people and organizations throughout history.