TLA

TLA is the three-letter acronym for "three-letter acronym." The fact that TLA is itself a TLA delights some folks far more than is seemly.

There are only 17,576 possible TLAs in the English language.

For the extremely geeky there are ETLA's (Extented TLAs) and EETLAs (Extened, Expanded TLAs).

Examples of TLAs in the Shadow Unit universe include AOP, ECR, and of course WTF.