Alternate history is, simply put: speculation about how history would've gone if an event (or multiple events) went differently than in our timeline.
The aim of said speculation is to provide a better understanding of our timeline, and to stress the importance of certain events — some of which have been relegated to mere footnotes of history, despite being so influential. It's also a good way to waste time.
Feel free to suggest additions!
The Atlantropa Articles by Cody Franklin
I Am Uncle Sam by Dean Mosley
Alternate History Hub