In alternate history terms, POD stands for Point of Divergence.
A POD is the point in history where a difference in a key decision or route taken leads to a completely different - alternate - version of history. It's the breakpoint where alternate history splits off from OTL history.
For example, in Harry Turtledove's Southern Victory series, the alternate history of a USA and CSA existing side by side came about because of a POD in the fall of 1862, when the Confederates recovered Robert E. Lee's orders vital to the campaign. In OTL, the orders were never recovered, and the Union army managed to stop Lee's campaign. In this timeline, the Confederates found the orders and won a victory that they never won in OTL. Thus, the recovery of the lost orders was the point of divergence.