Seeing
Also called: atmospheric seeing, twinkling, steadiness.
How steady the air is. Good seeing gives sharp, still views at high power; bad seeing makes stars boil and twinkle.
Pockets of warmer and cooler air bend starlight slightly differently, so stars twinkle and planets shimmer. In good seeing a star at high power is a tight point with a steady ring around it; in poor seeing it's a boiling blob. Seeing matters most for planets, the Moon and close double stars — and much less for faint, low-power deep-sky views.
It's caused by turbulence at all heights: the jet stream high up, wind over hills, and heat rising from roofs, roads and the telescope itself. Seeing and transparency are different things — crystal-clear nights are often turbulent.
At the eyepiece
Watch a bright star at high power: if it's steady, go for planets and close doubles; if it boils, switch to low-power deep-sky targets.
In AstroPilot
Hour by hour on Tonight rates seeing from 1 (poor) to 5 (excellent), estimated from jet-stream winds and wind shear.