Solar variability is a fundamental concept in heliophysics and solar-terrestrial physics that describes the temporal changes in the Sun's radiative, particulate, and magnetic outputs. It encompasses the investigation of fluctuations in total and spectral solar irradiance, the solar wind, energetic particle fluxes, and the solar magnetic field over timescales ranging from minutes to millennia, including prominent cycles like the approximately 11-year solar cycle. Research in solar variability is essential for understanding its drivers, its impact on the heliosphere and planetary environments, and its influence on Earth's climate and technological systems.