A term added by Albert Einstein as a modification to his original theory of general relativity, in order to balance the attractive force of gravity and achieve a static or stationary universe. It represents the possibility that there is a density and pressure associated with apparently empty space, and that the overall mass-energy of the universe is actually much greater than currently estimated. Once dismissed as just a mathematical “fix”, it has been revived in recent years with the discovery of the apparent acceleration of the expansion of the universe.