The Cosmological Constant is a term in <a href="/article/einstein-field-equations">Einstein's field equations</a> that represents the energy density of empty space. Initially introduced by <a href="/article/albert-einstein">Albert Einstein</a> to achieve a static universe, it was later discarded, only to be revived to explain the accelerating expansion of the universe, acting as a form of <a href="/article/dark-energy">dark energy</a>. It remains one of the biggest mysteries in modern physics.