A constant introduced by Einstein(1917) into the equations of general relativity to allow a steady state cosmological solution to the Einstein field equations. The constant was introduced before the concept of the Big Bang had been conceived, so an expanding or contracting universe was regarded as physically implausible, leading Einstein to add as a "fudge factor." In theory, the constant can be derived from quantum field theory, but the derivation has not yet been performed. Einstein's cosmological constant is equivalent to a vacuum energy density, which means it can be put on the left hand side of Einstein's equations with the geometry (as Einstein did), or on the right hand side with the stress-energy, both forms being mathematically equivalent.
Generally, does not play a significant role in the early universe.