A person or animal that runs away or has run away; a person, animal, or organization that escapes captivity or restrictions.

“Runaway children are vulnerable to criminal exploitation.”

A vehicle (especially, a train) that is out of control.

(usually attributive) An object or process that is out of control or out of equilibrium.

Having run away; escaped; fugitive

“a runaway thief”

Easily won, as a contest

“a runaway victory at the polls”

Unchecked; rampant

“runaway prices”