An eseeka was an Argrathi geometric art form. It was formed by drawing a circle and allowing the circle to encompass one's mind completely, which allowed the resulting art to flow from the brain.
While Miles O'Brien was in the simulated Argrathi "prison" for twenty years, his companion, Ee'char, showed him how to draw eseekas. O'Brien was skeptical at first, but as time passed by, he learned patience as Ee'char had and was able to form eseekas.
Later, when O'Brien was in the real world and Ee'char was still with him, the eseeka became a symbol of O'Brien's developing insanity. (DS9 Hard Time)