Anything that gives people some form of joy. Fun is not repetitive, as too much of a good thing loses its effect. Animals are also known to have fun, as anyone who's had a pet knows.
It's easier to have fun while with others. When enough people are having fun, this is called a party. But in life, parties are rare, as humanity has yet to come up with a way to synchronize work and fun in such a way as to make a kid's favorite utopia possible.
There are a vast number of ways people can have fun, but this comes at a cost— literally!
Fun is a core part of almost every game; many even consider fun
While they can certainly vary in quality, video games also vary depending on hardware limitations, though technology became less restrictive in this regard.
One of the definitions says funny is "difficult to understand". Well that sucks.