An x-board is a kind of calendar used in military aviation training schools. Student pilots are required to complete a certain number of training missions (including flight simulator and aircraft missions.) Each time a student pilot completes a mission, he or she puts an "X" on the board (calendar) signifying the successful completion of the mission.
X-boards fill both a motivational and an organizational role in flight training. By using the x-board, students and instructors can quickly apprise themseves about the student's course progress, and therefore, students can have a sense of their advancement though challenging courses.