Short answers, pulled from the story.
Temperature is a physical quantity that reflects the average kinetic energy of the particles in a system, giving a numerical measure of hotness or coldness. It is measured with a thermometer and is important across physics, chemistry, biology, astronomy, and everyday life.
Absolute zero is 0 kelvin, equal to minus 273.15 degrees Celsius or minus 459.67 degrees Fahrenheit, and it is the lowest point on the thermodynamic temperature scale. At absolute zero no energy can be removed from matter as heat, a fact expressed in the third law of thermodynamics.
On the Celsius scale water freezes at 0 and boils at 100 at sea-level pressure, while on the Fahrenheit scale water freezes at 32 and boils at 212. The Kelvin scale is the absolute scale used in science, with its zero at absolute zero, and one kelvin is exactly equal in size to one degree Celsius.
The kelvin has been defined through particle kinetic theory and statistical mechanics since May 2019, with its magnitude fixed by a conventionally chosen value of the Boltzmann constant. Before that, from 1954, it was defined as a thermodynamic temperature using absolute zero and the triple point of water.
The lowest temperature ever attained by experiment is 38 picokelvin, which is 38 trillionths of a kelvin. The lowest temperature ever obtained in a macroscopic system was 20 nanokelvin, achieved in 1995 at NIST.
Lord Kelvin postulated absolute temperature in work published in 1848, building on the earlier work of Carnot. His definitive publication setting out the definition was printed in 1853, from a paper read in 1851.
On scales not referenced to absolute zero, a negative temperature is simply one below the zero point of the scale, such as dry ice at minus 78.5 degrees Celsius. On the thermodynamic scale, a spin subsystem can reach a negative temperature that is hotter than any positive temperature, though kinetic theory forbids negative temperatures for ordinary matter.