Final answer:
Absolute zero is the theoretical lowest temperature corresponding to -273.15°C on the Celsius scale and 0 K on the Kelvin scale, where all molecular motion stops.
Step-by-step explanation:
The temperature on the Celsius scale where all molecular activity stops is known as absolute zero. Absolute zero represents the lowest possible temperature where all molecular motion ceases. This point is observed at -273.15°C on the Celsius scale. In the Kelvin temperature scale, this is expressed as 0 K (kelvins), a scale commonly used in scientific measurements, particularly in astronomy. It's important to note that temperatures on the Kelvin scale are referred to as kelvins, not 'kelvin degrees'.
At this temperature, substances would exhibit no thermal energy and gases would have no pressure due to the lack of kinetic energy among their particles. Although absolute zero is a theoretical limit and cannot actually be reached, it helps define the Kelvin scale and serves as a reference point for thermodynamic temperature measurement.