The Cosmological Constant (Λ) is a term in Einstein's general theory of relativity to counteract the inward force of Gravity on the universe. Given his theory, it provided a means that the universe could remain static despite the pull of gravity.
The term has been revived because the concept is much like Dark Energy, an outward force, but as observed, dark energy is not constant.
General Relativity (GR)
Lambda-CDM model (ΛCDM)