The Center of Pressure (or CoP) is the point on an aircraft where all of the aerodynamic forces act. It is found by adding up the individual forces from the wings, tail surfaces, drag, etc. The CoP often moves with changes in attitude or speed, leading to the need for trim in order to keep the aircraft flying level.
The CoP is closely related to the center of gravity as it is the aerodynamic forces that are counteracting the static force of the CoG. A great amount of effort goes into keeping the two as close as possible.