In database theory,
ACID stands for:
Atomicity,
Consistency,
Isolation,
Durability.
Atomicity means that updates to the database must be "atomic" -- all-or-nothing. If you change a half-dozen individual rows, but the changes are conceptually related, they must all appear at the same time. Even if the power goes out or the eschaton comes, you mustn't commit only part of a change.
Consistency means that changes must be, well, consistent. If you change something in one place, and other values are dependent upon it, the change must either be disallowed or cascaded through those dependent values. Database changes must either be valid, or forbidden.
Isolation means that multiple users should be able to work on the same data in a reasonably intelligent manner, each independently. Suppose you're updating a catalog, where each row represents one item. Down the hall, someone else is working on the same item, in the same catalog. She commits her change to the description of the item. A few minutes later, you commit your change to the price of the item. You shouldn't be able to overwrite her change to the item's description. (Heck, you shouldn't be able to overwrite her change to the description even if you changed that element yourself - the database should stop you, pointing out the fact that something has gone horribly wrong here.) Changes that are not yet committed must not interfere with others' changes.
Durability means that, regardless of what happens, you should be able to maintain the database in a correct state. Even if someone pulls the plug, the database should be able to be restored to the state of the last successfully committed transaction. Once committed, a database update must never be lost.
E2 is basically a big database; I can't believe this wasn't noded before now. Weird.