If there is a disparity between how great you think you are at tennis and how you actually play, you are probably surprised by how often you lose. Disparity is the condition of being unequal, and a disparity is a noticeable difference.
Disparity usually refers to a difference that is unfair: economic disparities exist among ethnic groups, there is a disparity between what men and women earn in the same job. This noun derives from Latin dispar "unequal." The opposite of disparity is parity, the condition of being equal or the same.