Free-space optical communication is a method of data transmission that utilizes modulated light beams propagating through the atmosphere or outer space as the unguided medium. As a research field, it investigates the principles, technologies, and performance characteristics of establishing line-of-sight optical links for communication purposes. This encompasses studies of atmospheric propagation effects, beam control, modulation techniques, system design, and network integration, highlighting its significance as a potential high-bandwidth, rapidly deployable wireless alternative or complement to conventional guided or radio frequency systems in various terrestrial and extraterrestrial scenarios.