1. Games. A playing card whose value can vary as determined by its holder.
2. Sports. An athlete or team selected to compete in a tournament or playoff from among those who did not qualify by meeting the regular requirements.
3. Computer Science. A symbol that stands for one or more unspecified characters, used especially in searching text and in selecting multiple files or directories.
4. Slang. An unpredictable or unforeseeable factor: A surprise witness proved to be the wild card at the trial.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.