The lowest possible temperature, or absolute zero as it is called, is −273.15 degrees Celsius, or −459.67 degrees Fahrenheit. It is also called 0 degrees Kelvin, a temperature scale with increments equivalent to degrees of Celsius, but uses absolute zero rather than water's freezing point as its starting point. The lowest possible temperature is defined as the point at which all atomic motion ceases.

The above definition may be incomplete, however, as an atom is itself an entity with complex internal structure. To achieve the lowest possible temperature, or true absolute zero, not only must atomic motion stop, but all of the atom's internal components would need to stop as well. Electrons would need to stop orbiting their respective atomic nuclei, the neutrons and protons in the nuclei would need to stop pulling each other around with their internal forces, the quarks, and any underlying substructure must cease all activity. Due to quantum mechanical effects, this is impossible. Thus, a more precise definition of the lowest possible temperature applies to collections of matter from which no further thermal energy may be extracted, i.e., another collection of atoms brought into contact with the sample will always transfer energy to it, never the reverse.

Like the efficiency of a system, the velocity of a particle, or the maximum possible temperature, absolute zero is actually a theoretical quantity which can only be approached, but never achieved.

Temperatures near absolute zero have been achieved with the techniques of laser cooling and magnetic evaporative cooling. In laser cooling, fast-moving atoms are jostled with photons until they slow down to 1/10,000th of a degree Kelvin. In magnetic evaporative cooling, the remaining atoms are held in loosely place by a magnetic field, and the more energetic atoms eventually escape, leaving behind the slowest remainders. Using these techniques, temperatures as low as 250 picoKelvins (pK) have been achieved. At these low temperatures, matter can behave in bizarre ways, forming structures called Bose-Einstein condensates, which demonstrate a property called superfluidity, or the flowing of atoms without viscosity.