The steradian is the unit of solid angle, defined as the ratio of the projected area subtended by a surface to the square of the distance from the observation point. It is a unit of measure in optics and astronomy that quantifies the amount of visual angle subtended by an object, providing a two-dimensional angular measure of light flux density. Since it represents a ratio of area to the square of a distance, it is considered dimensionless within certain systems of physical units, making it a fundamental quantity in the description of radiation patterns and angular intensity measurements across various physical sciences.