Hydrostatic equilibrium is a state in which a fluid, such as water or air, is at rest and experiences no net movement. In this condition, the gravitational force pulling the fluid downward is balanced by the pressure gradient force pushing upward. This balance allows the fluid to maintain a stable shape and prevents it from collapsing or expanding uncontrollably. In the context of celestial bodies like stars and planets, hydrostatic equilibrium is crucial for maintaining their structure. For example, in a star, the inward pull of gravity is countered by the outward pressure from nuclear fusion reactions in its core, allowing it to remain stable over time.