The moment the sun goes down and disappears below the horizon — the same thing as 'sunset'. The difference is feel, not meaning. 'Sunset' is the everyday word and is also what you call the beautiful red sky itself ('look at that sunset!'). 'Sundown' points at the *time* rather than the view, and it carries a slightly old, American, rural flavour — cowboy films, farm work, 'be home by sundown'. It also turns up in rules and religious contexts where a deadline matters: many Jewish and Muslim observances are counted from sundown to sundown. It is normally uncountable and used without 'the': 'at sundown', 'before sundown', 'after sundown'. There is a verb 'to sundown' in older or technical use, but you will not need it and should not use it.