"Seeing" is the term astronomers use to describe the stability of the atmosphere for observing. Seeing significantly affects both the appearance of stars and of extended objects like planets, galaxies, and nebulas, but it also can affect all astronomical and terrestrial, day and nighttime, visual, and photographic observations.
Air can bend light just like a lens, distorting rays of light from distant objects. It does this because of different air temperatures and densities in the path of the light through the atmosphere. Because air is constantly in motion, the amount of blurring or smearing changes from moment to moment.
When there is little change or distortion, this is called good seeing. Fine detail is visible on the brighter planets like Jupiter and Mars, while stars are pinpoint images. When there is a lot of change and distortion, it’s called bad or poor seeing. Images are blurred and star images are diffuse. Seeing conditions can be numerically rated on a five point scale, where one is worst and five is best.
Terrestrial uses--birdwatching, harbor and ocean patrols, surveillance, and target shooting--are heavily impacted by seeing. These activities almost always occur in daytime and are usually done with a nearly horizontal line of sight, so effects of surface heating and turbulence are at a maximum and can significantly degrade both viewing and photography.