What is a zillion?

The boys always ask about high numbers and wonder what number is the biggest. They'll ask, "Mom, is 80-thousand-million-200, is that the biggest number?" and other variations of this question. I found a TV program called, "How Much is a Million?" & thought I'd let them watch it. It compares a million, billion, trillion, and zillion by telling how far that number of children would reach if they were stacked up, how much water would be necessary for that many goldfish, how long it takes to count to that number, etc. So they watched this on Saturday morning and after, Nathaniel asked, "Mom, what is a zillion? I have never heard of that. What is it?" I started to plan my explanation, when his Dora & Diego kicked in and he continued, "they must be trying to talk Spanish."