A Deck out is a loss due to having no cards in the Main Deck when that player would draw a card. Mill Decks, such as "Laval", use this as their primary victory condition.
A Deck out can only occur when a card would necessitate drawing specifically; being unable to reveal (including excavate), look, add, send, banish, etc. cards from the Deck will not result in a loss.
In the anime
In the anime, a Deck out has occurred only three times: Seto Kaiba decked out against Maximillion Pegasus, Strings against Yami Yugi, and Howard X Miller against Jaden Yuki.
Duelists such as Yami Bakura, Howard X Miller, Sartorius, Mr. Armstrong, Breo, Vector and Eliphas have attempted to cause their opponents to deck out, albeit unsuccessfully.