1. sports The symbols used to denote the players on one's team (and usually those of the opposing team) in a diagram of a play (i.e., the formation the players will take when trying to score or move the ball). The term often refers to the plays themselves. That coach is great with the x's and o's, but the team's execution has been very disappointing.

2. By extension, knowledge of how something works. Before you start on that report, call Shelly—she knows all the x's and o's of the budget.