I prefer the definition of knowledge given by Orson Scott Card, if I'm not mistaken: something you believe to be true. It doesn't have to be justified and it doesn't even have to be true - you just have to believe it.

I know that the Moon is about 400,000 km away from the Earth. I have never measured the distance and I haven't even tried to check if it's plausible: I read it somewhere and accepted it as such.