LDS definition: Satan, also called the adversary or the devil, is the enemy of all righteousness and of those who seek to follow God. He is a spirit son of God who was once an angel "in authority in the presence of God" (D&C 76:25; see also Isaiah 14:12; D&C 76:26–27). But in the premortal Council in Heaven, Lucifer, as Satan was then called, rebelled against God. Since that time, he has sought to destroy the children of God on the earth and to make them miserable.
Wikipedia on 'the Devil': Devil
The actors that portrayed Satan in the first (pre 1990) and second (1990-2013) temple films. Derryl Yeager on the left and Michael Ballam on the right.