A megaturn strategy is one that eschews buying any Victory cards until one last game-ending turn. In contrast with a typical Big Money strategy, where you buy one Victory card per turn throughout the endgame, a megaturn deck will buy no Victory cards for the first 11-14 turns of the game and then buy enough Victory cards to end the game in a single turn, at some point between turns 12 and 16. Common enablers of megaturn strategies are King's Court, which super-charges engines, and Bridge, which provides cost reduction and +Buy. In fact, King's Court and Bridge and Native Village and Bridge are powerful combos that can be used to set off a megaturn.
Megaturn strategies often involve the purchase of cards that would normally break a deck if purchased gradually. When pursuing a megaturn strategy, you do not need to consider whether or not your deck is resilient to greening, as you can end the game on the same turn you buy most of your Victory cards.