Declination is one of the two rotational axes used by astronomers, and also by game programmers. Declination is similar to pitch-- if you nod your head "yes," you are rotating in declination.
Most engines count from +90 to -90 degrees, starting out pointed straight up, turning downwards and ending pointed straight down. Straight forward is 0 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 azimuth), because they do not use roll. This is similar to the way most FPS games work. It is preferred in some RT3D engines because it keeps the viewer level no matter where it points. But this system has a drawback because it suffers from gimbal lock whenever the viewer points either straight up or straight down.