After much deliberation, I have come to a point where I feel like I can adequately describe what an engine is in layman's terms.
An engine is a deck where you are able to both (a) potentially gain more than one set of points a turn and (b) seeks to increase consistency (or at least not decrease consistency) in the deck before greening. In this way, an engine seeks to expand further than a set boundary of one Province/Colony/Dominate a turn.
Let me break down what that means a bit. Say you buy an Estate. That is a set of points, or in this case a point. Say you buy a Province, that is a set of points. Any time you score with a purchase or gain, that is a single set of points, no matter how big or small. Play a Monument? That is a set of points. Play a Goons and buy something? That is a set of points. Buy a copper and get Fountain points? You get the picture.
Secondly, consistency increase in deck building, at least to the point of where consistency does not decrease. The idea is, you want to play your good cards more often. This means trashing, this means draw, this means sifting.
Since a "money" deck seeks to only buy a single set of points a turn, thusly it only has an X hurdle to clear. Engines have a higher hurdle to clear and thusly want at least the potential for an increase of consistency, and an output greater than a single set of points a turn.
Some edgecases:
1. Workshop/Gardens. This deck does gain more than one set of points a turn, however it does not increase consistency and thusly is not an engine.
2. Donate with Lucky Coin, a lot of action cards, but no other gains. This deck will be highly consistent, however it will have no cap larger than a single set of points, and thusly there is no engine on the board.
3. An engine that is possible given the rules, but can only gain estates from Workshop/Ironworks. This is indeed an engine because it will increase consistency and score more than one set of points, however it is a very weak engine, and may lose to the more focused money deck.
4. Council Room/Margrave deck with nothing else. Yes, this is an engine. It increases consistency and it can score more than one set of points a turn. However, it is a very weak engine.
5. A deck with Lurker, Graverobber, Chapel, and no money at all but no Action increasers: The idea here is you Graverob something every turn, score a single Province, and reget your trashed stuff every turn with Lurkers, while getting incredibly consistent with the Chapel. Yes, this is merely "money", it is not an engine since there does not exist a way to score more than one set of points a turn.
I might be missing something here, but that is my current theory on engines. As you can see, the terms "money" don't always naturally fit (due to case 5 as an example), and thusly I am open to ditching the term entirely and going with a new one, to entirely replace the concept of money, except in terms of referring to the really bad single draw card Big Money of ages past.