A mondegreen is the mishearing or misinterpretation of a oft-heard phrase (usually something like a line in a poem or a song lyric), due to near homophony,* in a way that gives it a new meaning.
"Our Father who art in Heaven, Harold be his name" for
"Our Father who art in Heaven, hallowed be his name"
The term mondegreen was coined by Sylvia Wright in a 1954 Atlantic article. As a child Sylvia had listened to a folk song that included the lines "They had slain the Earl of Moray / And laid him on the green," which she'd misheard as "They had slain the Earl of Moray / And Lady Mondegreen."
* the linguistic phenomenon whereby words of different origins become identical in pronunciation