A person who fixes bicycles. Can usually be spotted by the inordinate amounts of black grease deposited under their fingernails and skin, and on their ragged Fox Racing T-shirts. Other signs of a bike mechanic are clipless pedal shoes, spotted by the metallic *click* that the cleats make and jewellery made of bike chains, derailleur hangers, etc. Bike mechanics are like the monks of the biking world, much revered but usually poor.
But with enough years of training and experience mechanics can reach the holy grail of the Team Mechanic, where you'll fix bikes for the likes of Cannondale-Volvo, Trek-Volkswagen, Kona-Ford, and many other revered teams. By this point mechanics usually specialize into wheelbuilding, drivetrains, bikebuilding, etc.
To become a bike mechanic you'll need to know what all those parts are named, what they do, how to fix them, and lots of tools. There are many books out there that can help you achieve this goal. One good node on this subject is Everything I know about bicycles and cycling.