Acronym for "Atomicity, Consistency, Isolation, Durability", as understood in data-management theory (which underlies engineering of database management systems DBMSs).
Accessible introductions to the value of ACID include ones from
As Donal Fellows said in The comp.lang.tcl newsgroup:
The Holy Four Properties of Transactions, which are (IIRC): Atomic - it happens or it doesn't Consistent - the story doesn't change underneath your feet Isolated - other people can't mess you up Durable - once you've finished commit, it doesn't go away It's a shame there's not three of them, or we could make religious jokes. :^)
DGP: That's not really a good definition of Consistent, even informally. Better might be: "you can't $#^&#$&^ it up." Important thing is that invalid transactions are not permitted. CL adds: the issue with consistency most often is that a particular datamanager doesn't support consistency semantically; it's not particularly that extant datamanagers are wont to behave erratically (at least in this regard). I repeat: SQL-less datamanagers often are not able to express "consistency" as powerfully as SQL can.
[Explain why and how MetaKit and MySQL don't respect ACIDity.]
See also the Acronym collection