Twilight is the illumination that is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere when the Sun itself is not directly visible because it is below the horizon, so that the surface of the Earth is neither completely lit nor completely dark. The word "twilight" is also used to denote the periods of time when this illumination occurs.

The further the Sun is below the horizon, the dimmer the twilight (other things such as atmospheric conditions being equal). When the Sun reaches 18 degrees below the horizon, twilight's brightness is nearly zero. Evening twilight ends and night begins. When the Sun again reaches 18 degrees below the horizon, night ends and morning twilight begins. Owing to its distinctive quality, primarily the absence of shadows and the appearance of objects silhouetted against the bright sky, twilight has long been popular with photographers, who refer to it as Sweet Light, and painters, who refer to it as the blue hour, after the French expression l'heure bleue.

By analogy with evening twilight, the word "twilight" is also sometimes used metaphorically, to imply that something is losing strength and approaching its end. For example, a very old man may be said to be in the twilight of his life. The collateral adjective for twilight is crepuscular; it is most frequently encountered when applied to certain insects, fishes, and mammals that are most active during that time.