Zernike polynomials are mathematical blueprints of ocular aberrations measured with wavefront technology. Each Zernike polynomial, called a mode, describes a certain type of geometric shape, a certain three-dimensional surface. The second-order Zernike terms represent the conventional aberrations defocus (spherical correction) and astigmatism. Zernike aberrations above the second order are called higher order aberrations. The third-order Zernike terms are coma (a wavefront shape with twofold symmetry) and trefoil (a wavefront shape with threefold symmetry). The fourth order Zernike terms include spherical aberration and four other terms, and so on. The Zernike polynomials are an infinite set, but in ophthalmology discussion has generally been limited to the first ten or so.