What does AD&D stand for Dungeons and Dragons?

In 1977, the game was split into two branches: the relatively rules-light game system of basic Dungeons & Dragons, and the more structured, rules-heavy game system of Advanced Dungeons & Dragons (abbreviated as AD&D).

https://en.wikipedia.org/wiki/Dungeons_%26_Dragons#:~:text=In%201977%2C%20the%20game%20was,Dragons%20(abbreviated%20as%20AD%26D).