wild card

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun In card games, a card that can be assigned any value or used to substitute for any needed card.
• noun sports A competitor or team that is allowed to compete in a tournament despite not reaching the normal standards to qualify for inclusion.
• noun computing a character that substitutes for other characters in regular expressions and globbing

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a playing card whose value can be determined by the person who holds it
• noun an unpredictable factor