FFL
Federal Firearms License — required to engage in the business of dealing or manufacturing firearms.
An FFL is a federally-issued license required for anyone in the business of selling, manufacturing, or importing firearms. Several types exist: 01 (dealer), 03 (curio and relic collector), 07 (manufacturer), and others. Across-state-lines firearm transfers must go through an FFL, with a Form 4473 and a NICS check.