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The cosmological constant (Λ) is a term in Einstein's field equation (general relativity's equation) which counteracts the inward force of gravity on the universe. Given general relativity, the term provides a means by which the universe could remain static despite the pull of gravity. Einstein dropped the term when the universe was shown to be other than static, i.e., expanding.
The term has been revived because the concept corresponds to dark energy, a presumed outward force to explain the universe's more-recently-discovered expansion-history, which does not quite fit gravity's prediction. However, dark energy, as deduced from observation, is not constant.