A person who wears black suits and sunglasses that investigates federal, local, or worldly matters. Most of them are stiff, drive black cars, drink lots of coffee, make over $100,000 a year, wear casual designer-type clothes when not at work , travel all over the world, use lots of technology, act very secretive and quiet, and are masters of disguies.
Some of them have desk jobs, while the majority work in the field. Sometimes, they have to go undercover. At work they are called Agents, and non-FBI agents are referred to as 'civilians'. At home, they live regular lives, just like you and me.
Girlfriend: Darius, where have you been? You've been gone for 2 weeks now!
FBI Agent: I can't tell you that. That information is classified.
Girlfriend: Oh, come on!