Basically, absolute zero is the lowest possible temperature where nothing could be colder and no heat energy remains in a substance. Absolute zero is the point at which atoms stop vibrating, or in more scientific terms "the fundamental particles of nature have minimal vibrational motion, retaining only quantum mechanical, zero-point energy-induced particle motion."
By international agreement, absolute zero is defined as precisely:
* 0 K on the Kelvin scale, which is a thermodynamic (absolute) temperature scale, and
* –273.15 °C on the Celsius scale.
Absolute zero is also precisely equivalent to…
* 0 °R on the Rankine scale (also a thermodynamic temperature scale although I have never heard of it, similar to Kelvin i guess), and
* –459.67 °F on the Fahrenheit scale (slight disagreement to BhajunSingh's post, but this measurement came from Wikipedia

)
While scientists can not fully achieve a state of “zero” heat energy in a substance, they have made great advancements in achieving temperatures ever closer to absolute zero (where matter exhibits odd quantum effects). In 1994, the NIST achieved a record cold temperature of 700 nK (billionths of a kelvin). In 2003, researchers at MIT eclipsed this with a new record of 450 pK (0.45 nK). NIST did similar things with their caesium fountain atomic clock project, cooling atoms down to almost absolute zero.
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