The cosmological constant is a parameter introduced in 1917 by Albert Einstein into the description of the universe in terms of the general theory of relativity, in order to ensure static (i.e., not expanding or contracting) cosmological models. At that time, the expansion of the universe was still unknown. Einstein later described the invention of the cosmological constant as his greatest blunder. Einstein's cosmological constant was later interpreted as the energy density of the vacuum. The observational upper bound on the value of the vacuum energy density is 40 to 120 orders of magnitude smaller than that predicted from quantum field theories of elementary particles – the so-called cosmological constant problem. The cosmological constant may be the cause of the acceleration of the universe recently inferred from observations of Type Ia supernovae, but again there is as yet no theoretical understanding of why it would have the small, non-zero value needed to explain these observations.