1. A person who habitually uses small, hot-shoe mountable electronic flashguns (or speedilights), off-camera to enhance the lighting quality of his or her photos. The off-camera flashgun is triggered by the camera using a sync cable or a wireless switch.
2a. More generally anyone who attempts to generate professional quality photographs by using portable, lightweight and (relatively) inexpensive lighting equipment, creatively.
2b. The philosophy that a person described in 2a. espouses.
4. The eponymous website that disseminates these ideas about light and lighting to anyone who is interested.
Definition 1: Bob's a strobist so his photos never have that DMV (Department of Motor Vehicles), mugshot feel to them.
Definition 2a: Tomorrow's photo shoot is only for the fleetfooted so I plan to use exclusively strobist gear.
Definition 2b: The moment I saw the strobist touch Rick had given to his tiny studio, I knew how he was getting his amazing results.
Definition 4: If you think you will make the cover with that gear, you better be hitting the strobist site some time soon.