I don't think there's any one given engine people are always referring to (got any links or examples?).
Generally when someone refers to an engine in a deck they're referring to whatever combination of cards makes the deck generate enough money/cards/VP to move towards winning every turn.
For example of general engines:
-5+ Minions in a deck gives you a Minion engine. They will combine with each other to create money and to cycle through your deck so you can find good cards (or more Minions) to play over and over again in order to win.
-Several King's Courts plus cards like Grand Market, City, Market, or Peddler will give you money, actions, and cards to draw MORE King's Courts and good cards. This creates an engine where you play a billion actions every turn and end up drawing your deck and buying multiple victory cards.
-A super thin deck (thanks to Chapel, Remake, etc) with a few cycling cards (Village, Great Hall, Hamlet, Pawn, etc) and a bunch of Conspirators will draw a bunch of cards every turn while giving you a bunch of money to buy Province/Colony. This would be a Conspirator engine.
-A deck with lots of +Action, some +Cards and a bunch of Goons will ignore buying green cards and try to win the game buying many cheap cards for +3/4/5 VP chips each every turn. This would be an example of a Goons engine.
EDIT: ...And apparently WW gave the exact same examples a minute or two quicker. Damn my elaboration.