Cosmological constant
A term Einstein added to his equations to keep the universe static — later reportedly his “biggest blunder.”
In 1917 Einstein added a constant (Λ) to his field equations so they would describe a static universe. When Hubble's observations (1929) showed the universe is expanding, Einstein dropped the term. According to the physicist George Gamow — the only source for the quote — he called it his “biggest blunder.”
The twist: since supernova observations in 1998 revealed that the expansion is accelerating, the constant has returned to represent dark energy, the mysterious cause of the universe's accelerating expansion.
Source: Wikipedia: Cosmological constant — Read the full article on Wikipedia.