What is Mormonism?

There are Mormons, lots of them, but Mormonism, an abstract idea, cannot be well-defined. 

You can say "It is an American religion, begun by Joseph Smith," but this quickly became many religions (and many Churches) after his death in 1844. 

The largest Mormon church now is the LDS church in Utah, and the second largest the RLDS (now the Community of Christ) in Missouri. 

This is the Church of my family, and it has a problem with Mormonism. Is it, or is it not a Mormon church? It cannot make up its mind.