A K3 surface is a special type of complex surface in algebraic geometry. It is defined as a smooth, compact surface with a trivial canonical bundle, meaning it has no "twisting" in its geometry. K3 surfaces are important in various areas of mathematics, including string theory and mirror symmetry. These surfaces can be thought of as higher-dimensional analogs of Riemann surfaces. They have rich geometric structures and can be described using complex coordinates. K3 surfaces are also known for their unique properties, such as having a finite number of rational points and being simply connected.