We all know what alternate history is, despite it (oddly) not being included in AH Tropes 101. Some of us are writing it; all of us are reading about it.
But what exactly is history?
His Despotic Majesty isn't deigning to try and define it here. He instead is settling for a series of aphoristic thoughts on the matter, which hopefully strike a balance between thought-provoking and utterly useless.
Let's start with an obvious one. History is the springboard from which alternate histories are launched. When we write alternate history, we're acknowledging that it didn't actually happen.
...happening all the time.
This is at once a blessing and curse of history: from an AH writer's perspective, there's so much of it to work with, and it doesn't go away simply by ignoring it. It also doesn't come into being simply because you wish it did.
At some point in the next thousand years, the exact second-by-second reconstruction of the death of Osama bin Laden will be accessible through FOIA requests. I have no doubt that when it does, there will be an AH or two whose POD involves slight changes in the trajectory of individual bullets fired by SEAL Team Six.
In OTL, the Protestant Reformation was set in motion by a monk who wasn't even thinking of taking vows until he was nearly hit by lightning. In OTL, British India could've been derailed because a single guy put a pistol to his head, pulled the trigger, and had the gun misfire. Twice. In OTL, a single demagogue survived 40 assassination attempts before dying by his own hand while the Russians closed in.
It takes deliberately bad writing to be less plausible than what actually happened.