Albus
Albus Dumbledore.
It doesn't stand for anything. It's the name of the headmaster of Hogwarts, Albus DUMBLEDORE.
The Hogwarts headmaster (Harry Potter books and films) is "Professor Albus Percival Wulfric Brian Dumbledore."
Albus Percival Wulfric Brian Dumbledore is the headmaster up until book seven when Severus Snape replaces him. After Snape it is unknown for sure who became headmaster but J.K. Rowling confirmed that McGonagall wasn't Headmistress.
Armando Dippet. Albus Dumbledore. Minerva McGonagall. (Severus Snape was Headmaster, too; but he was not elderly)
He was called Armando Dippet, and was the headmaster before Albus Dumbledore.
Professor Albus Dumbledore.
In Dumbledore's, the Headmaster's, office.
In the second book when Dumbledore has to leave for a while she steps up to the plate. Dumbledore comes back and is headmaster until he gets killed by Snape and again she becomes headmaster.
12
He was Professor of Transfiguration.
Dumbledore means "bumblebee" accoring to J.K. Rowling, which she thought was a good name and characteristic for Albus Dumbledore as he was constantly roaming about the castle just humming to himself.