Three Letter Abbreviation. ...
In gov't and military circles, "three letter acronym".
Our son spent three weeks with us last month during his break between State Department assignments. He was rolling his eyes at the propensity for every agency he works with to have their own sets of acronyms you'd never figure out, most being of the three-letter variety. I told him after one, "Gawd. Yet another TLA."
He looked at me funny and genuinely asked what a "TLA" was. He obviously laughed when I told him, with "Yeah. I'm gonna use that!"
Raised him right.