Definition of warrant:
(n) :
Authorization or certification; sanction, as given by a superior; something that provides assurance or confirmation; a guarantee or proof: a warrant of authenticity; a warrant for success
"he almost gives his failings as a warrant for his greatness (Garry Wills)."
(v) :
Justification for an action or a belief; grounds; to guarantee
"circumstances arose that warranted the use of lethal force."