Harley-Davidson is a motorcycle company.
Homer and Marge won a vintage 1955 Harley-Davidson at a dance competition in Greaser's Cafe. After Bart taught Homer to ride it, he formed a motorcycle gang called the Hell's Satans. Homer fought Meathook with it to win Marge back after Meathook kidnapped her.
Homer later rode the bike down the stairway of his house in Midnight Towboy.