NDA stands for "Non-Disclosure Agreement."
Journalists and other visitors to Blizzard and other game development studios are often required to sign such documents when they arrive, legally pledging themselves to keep secret the things they see for a certain time. Many beta testers have traditionally been held to fairly strict NDAs, though Blizzard has not included one with their beta tests in recent years.
It's expected that the Diablo III beta will not include an NDA, which means that fans can freely post screenshots, movies, and every sort of information taken from the beta.
- See other reference terms.