Heat Death of the Universe
Definition
The universe's far-future fate in current theory — everything spread so evenly that no usable energy remains, ever.
Technical definition
As entropy approaches its maximum in an expanding universe, temperature gradients vanish; with no gradients no work is possible (second law of thermodynamics).
Example
Picture the cosmos ~10¹⁰⁰ years out: black holes evaporated, particles decayed, a lukewarm dilute expanse with nothing left to power anything.
Common confusion
Heat death means maximum entropy, not maximum temperature — the universe ends cold and dilute, not in fire. 'Heat' here is about thermodynamics, not hotness.