A geometry shader is a type of shader used in computer graphics, specifically within the graphics pipeline. It processes geometric primitives like points, lines, and triangles, allowing for the creation of new geometry or modification of existing shapes. This shader operates after vertex shaders and before fragment shaders, enabling it to enhance the visual complexity of 3D models. Geometry shaders can generate additional vertices, which can be useful for effects like creating billboards or adding detail to surfaces. They are part of the OpenGL and DirectX graphics APIs, providing developers with tools to create more dynamic and visually appealing graphics in video games and simulations.