Altitude
Also called: elevation, height above the horizon.
How high an object is above the horizon, from 0° on the horizon to 90° straight overhead.
Altitude, with azimuth, tells you where to look. Low objects shine through much more air: they're dimmer (see extinction), redder and blurred by seeing. Most objects are best near their highest point, when they cross the meridian.
At the eyepiece
Measure altitude with your hand at arm's length: a fist is about 10°, so the horizon to overhead is about nine fists.
In AstroPilot
AstroPilot gives every object's altitude and direction, the hours when it's above your minimum altitude, and when it's highest.