Runaway hit is a term used to describe a
single which has not formally been released, which
becomes a hit on its own. As singles before 1998 were not
allowed to chart unless they had been formally released
commercially, runaway hits did not exist before 1998. The
most successful runaway hit is
The Black Eyed Peas' "My Humps", which became the first
unsolicited song to hit the Top 10 of the Billboard Hot 100.
Another example of a runaway hit is Beyonce's "Dangerously
in Love 2". The term is generally applied to unsolicited
songs which show a relative overall chart performance, not
songs which chart, but don't perform that well.
Nevertheless, the term has been applied to songs of this
caliber too, like Mariah Carey's "Mine Again", from
The Emancipation of Mimi (2005)