"Unchained Melody" is a 1955 song with music by Alex North and lyrics by Hy Zaret.
In 1955, Alex North used the music as a theme for the prison film Unchained, hence the name.
Todd Duncan sang the vocals for the film soundtrack. Les Baxter released an instrumental version
which reached #2 (though the Youtube video says #1). It's been remade a number of times since then.
(Source: Wikipedia)