absolute zero
Temperature is, at heart, a measure of how vigorously things jiggle. Cool a substance and the jiggling slows; cool it further and it slows more. Absolute zero is the floor of this descent — the coldest temperature there can be, where thermal motion drops to its irreducible minimum and there is simply no heat left to remove.
On the thermodynamic scale this floor sits at 0 kelvin, equal to about −273.15 °C. It is not merely 'very cold'; it is the natural zero of temperature, the point from which the whole kelvin scale counts upward. At this limit a system has been squeezed into its lowest-energy state, and (for a perfect crystal) its entropy reaches zero — the meeting place of the third law.
Why it matters: absolute zero anchors the temperature scale and gives thermodynamics a true bottom, the way sea level anchors altitude. But two careful caveats: first, you can approach it ever more closely yet never actually reach it (the third law forbids the last step). Second, motion does not entirely stop even there — quantum mechanics leaves a residual 'zero-point' jiggle that no cooling can ever still.
The coldest natural place known, the Boomerang Nebula, hovers near 1 K — colder than the leftover glow of the Big Bang at 2.7 K. Yet even there, atoms still tremble with zero-point motion. Laboratories have gone far colder still, into the nanokelvin range, but that last razor-thin sliver down to true 0 K stays forbidden.
0 kelvin ≈ −273.15 °C: approachable without limit, reachable never.
Absolute zero does not mean 'all motion stops.' Quantum zero-point energy keeps atoms in a faint perpetual quiver even at 0 K — the classical image of frozen-still particles is a useful but inexact picture.