An eclipse was, according to what Autloc once told Barbara, when the Moon passed before the sun and caused darkness.
Aztec tradition required that during an eclipse, a human sacrifice — indeed, the "Perfect Victim" — be given to the Sun God. (TV: "The Bride of Sacrifice")
When Tycho Brahe was fourteen, he witnessed a solar eclipse in Denmark. The fact that it was predicted inspired Brahe to become an astronomer. (COMIC: Herald of Madness)
In 1794 France, the First Doctor, as part of his plan to escape from a road works overseer, claimed that there would be an eclipse, distracting the overseer long enough for the Doctor to take some money from his purse. (TV: "Guests of Madame Guillotine")
The Tenth Doctor watched a solar eclipse in the Arctic in 2019. He found it was beautiful. (COMIC: Arctic Eclipse)
In 2400, the Daleks planned to use a solar eclipse in a plan to destroy Earth, but were defeated. (COMIC: Battle for the Moon)
All three suns of the Planet of Light were eclipsed once every three hundred years, resulting in a brief period of darkness which caused the inhabitants to go insane with fear and destroy their civilisation. (COMIC: World Without Night)