Azimuth is one of the two rotational axes used by the astronomers, and also sometimes by game programmers.
Azimuth is similar to yaw-- if you shake your head "no," you are rotating in azimuth. Most engines count from 0 to 360 degrees, starting out pointed straight forward, turning clockwise all the way around, and ending pointed straight forward again. Straight forward is both 0 and 360 degrees, and straight backwards is 180 degrees.
Sometimes mathematicians measure it in radians, a unit of measure that comes from the magical number Pi. A radian equals Pi/180, but most people use degrees instead of radians.
Astronomers use only two axes (the other axis is declination), because they do not use roll. This is similar to the way most FPS games work. It is preferred in some game engines because it keeps the viewer level no matter where he points. But this system has a drawback because it suffers from gimbal lock whenever the viewer points either straight up or straight down. The Quake games, for instance, avoid gimbal lock by not allowing the viewer to point all the way up or down.