Exit pupil
The width of the beam of light leaving the eyepiece: aperture ÷ magnification. It sets how bright and sharp the view looks.
Hold a telescope up to the daylight and you'll see a small bright disk floating behind the eyepiece: that's the exit pupil. A big one (5–7 mm) gives the brightest, widest views for faint nebulae — but never more than your eye's pupil (about 7 mm when young, 5–6 mm later in life). A small one (0.5–1 mm) gives high power for planets.
Exit pupil is the observer's secret weapon: the same exit pupil gives the same surface brightness in any telescope, so experienced observers choose eyepieces by it.
In AstroPilot
AstroPilot picks eyepieces by the exit pupil each kind of object needs, and the gear page shows which sweet spots your kit covers.