Dusk is the beginning of darkness in the evening

Evening

Evening is the period between the late afternoon and night when daylight is decreasing, around dinner time at 6pm. Though the term is subjective, evening is typically understood to begin before sunset, during the close of the standard business day and extend until nightfall, the beginning of night...

, and occurs after twilight

Twilight

Twilight is the time between dawn and sunrise or between sunset and dusk, during which sunlight scattering in the upper atmosphere illuminates the lower atmosphere, and the surface of the earth is neither completely lit nor completely dark. The sun itself is not directly visible because it is below...

, when the sky generally remains bright and blue. Civil dusk is when the earth has rotated enough that the center of the sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 is at 6° below the local horizon. This marks the end of the evening civil twilight, the point where artificial illumination is required to read outside.
It can be confused with sunset

Sunset

Sunset or sundown is the daily disappearance of the Sun below the horizon in the west as a result of Earth's rotation.The time of sunset is defined in astronomy as the moment the trailing edge of the Sun's disk disappears below the horizon in the west...

, which is the point at which the earth has rotated enough, that the sun is no longer visible from the local horizon.