The Slytherin Harry AU is an alternate timeline in which Harry Potter is sorted not into Gryffindor, but rather into Slytherin.
The point-of-divergence occurs during Harry's Sorting on 1 September 1991. Instead of thinking "Not Slytherin, not Slytherin" in answer to the Sorting Hat's question of where to place him, Harry thinks "Anywhere at all; anywhere I can belong". As a result, the Hat sorts him into Slytherin.
This AU is chronicled in the story Better Be Slytherin.