Absolute zero is the lowest possible temperature: −273.15°C (0 Kelvin). Temperature is related to the energy of particles. As matter cools, its thermal energy decreases. In a gas, for example, molecules move more slowly. At absolute zero, a system reaches its lowest possible energy state.
However, particles cannot become completely motionless in every quantum system. According to the Heisenberg uncertainty principle, a particle cannot have both an exact position and exact momentum. Even at the lowest energy state, some systems retain zero-point energy.
According to the third law of thermodynamics, absolute zero cannot be reached through a finite number of physical processes. Scientists have cooled matter to billionths of a degree above it, allowing them to study unusual quantum effects.
There are also negative-temperature states, created in special laboratory systems. Despite the name, they are not colder than absolute zero. In thermodynamic terms, they are actually hotter than any positive-temperature state. This is possible in systems with limited energy levels.