Term reluctantly added by Albert Einstein to his equations of general relativity in order to obtain a solution to the equations that described a static universe, as he believed it to be at the time.
The constant has the effect of a repulsive force that acts against the gravitational attraction of matter in the universe. When Einstein heard of the evidence that the universe is expanding, he called the introduction of the cosmological constant the "biggest blunder" of his life. Recent developments suggest that in the early universe there may well have been a cosmological constant with a nonzero value.