The pressure angle in gear design is the angle between the line of action, which is the line along which the force is transmitted between meshing gears, and the common tangent to the pitch circles of the gears. It significantly affects the gear's load capacity, noise, and vibration, with common values being 14.5°, 20°, and 25° for standard gears, where a larger pressure angle generally increases strength but also increases the radial load on bearings.