What arbitrary term did Einstein insert into his General Relativity equations, a modification later called his 'biggest blunder'?

Answer

The cosmological constant (\Lambda)

Einstein introduced the cosmological constant into his field equations specifically to force a static solution, counteracting the natural dynamic prediction of his own theory, because the prevailing scientific view favored an unchanging cosmos.

What arbitrary term did Einstein insert into his General Relativity equations, a modification later called his 'biggest blunder'?
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