Cosmological constant; Lambda
The constant introduced towards the Einstein field equation, planned to admit static cosmological solutions. During current philosophical view was the steady-state model of the Universe, where the Universe has been approximately for infinite time. Early analysis of the field equation denoted that general relativity allowed dynamic cosmological models only (ones that are either expanding or contracting), however no static models. Einstein introduced the most natural abberation to the field equation which he could think of: the adding up of a term proportional to the spacetime metric tensor, g, along the constant of proportionality being the cosmological constant:
G + Lambda g = 8 pi T.
Hubble's later on discovery of the expansion of the Universe mentioned that the introduction of the cosmological constant was needless; had Einstein supposed what his field equation was telling him, he could claimed the expansion of the Universe as possibly the greatest and most convincing prediction of general relativity; he called this the "greatest blunder of my life."