A practitioner of Agile Software Development who is fanatical about hewing to the Agile Manifesto.
Coined by J.F. Kelley for a Keynote Address given to the World Usability Day conference in Dayton, OH, November 2007.
(But was possibly also in use prior to that.)
Hey, if they can call their mission statement a "manifesto", I can call them "Agilistas"!