Cosmological Constant
Einstein's proposed energy density of empty space, currently the leading candidate for dark energy.
Videos Mentioning Cosmological Constant

Leonard Susskind on Richard Feynman, the Holographic Principle, and Unanswered Questions in Physics
Y Combinator
A term introduced by Einstein into his field equations of general relativity, representing energy density in space. Susskind explains it's essentially the same concept as dark energy and that its minuscule observed value is a major puzzle.

Andrew Strominger: Black Holes, Quantum Gravity, and Theoretical Physics | Lex Fridman Podcast #359
Lex Fridman
A term in Einstein's field equations that represents the energy density of empty space, now identified with dark energy, responsible for the universe's accelerated expansion.

Is Dark Energy made of Black Holes?
Sabine Hossenfelder
A constant of nature in Einstein's theory of general relativity that determines the curvature of empty space and is considered by some physicists to be equivalent to dark energy.

Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens | Lex Fridman Podcast #428
Lex Fridman
Einstein's proposed energy density of empty space, currently the leading candidate for dark energy.

Roger Penrose: Physics of Consciousness and the Infinite Universe | Lex Fridman Podcast #85
Lex Fridman
A term Einstein introduced into his field equations to allow for a static universe, later retracted but now significant in explaining the accelerated expansion of the universe.