Worldbuilding is the process of creating a fictional world. Tolkien's Middle Earth and C.S. Lewis's Narnia are examples of worldbuilding.
I've been working on a world called Materia for about the past decade or so. I think it started either in middle school or high school. It started off actually as a setting for a story I was writing about a couple of friends of mine, but then... It grew. A lot.
Materia has grown from just two countries on the same landmass to several continents. I've added races and taken away others. Countries have been created and destroyed at my whim, though I think I've started to settle on how I want the world to be arranged. One thing that hasn't changed much is its fantasy aspect, but I'm trying to keep some of the fantasy concepts more scientific.
I'm working on the details of my little planet, and each story is starting to be consistent with the major details, like the magic system and the politics of my countries. My current story, Wor(l)ds of Solitude, is also contributing to this process. I'm working on the religions on Materia as well as building more history into my world.
Hopefully, I'll finish most of the worldbuilding process eventually, and maybe I'll do another A-Z challenge next month to work on topics for Materia. That actually sounds like a good idea.