Cold

Cold is when the temperature is less than normal or the lack of heat.  Cold is a relative term used to describe the absence or low level of heat energy.  It is typically used to describe temperatures that are lower than what is considered normal or comfortable for a particular environment or living being.  What is considered cold can vary depending on a person's or organism's sensitivity to temperature, as well as cultural and geographical factors.

In physics, cold is defined as a low thermal energy state.  The temperature of a substance is a measure of the average kinetic energy of its molecules or atoms.  The lower the temperature, the less kinetic energy the particles have, and the colder the substance is considered to be.

Cold temperatures can have a variety of effects on living organisms, depending on their adaptations and tolerance levels.  Some organisms have evolved to live in very cold environments, such as the Arctic or Antarctic regions, while others cannot survive in temperatures that are too cold for their metabolic processes to function properly. 

In everyday life, cold temperatures can be uncomfortable or even dangerous for humans if proper precautions are not taken, such as wearing warm clothing and avoiding prolonged exposure to the cold.  Cold temperatures can also have practical applications, such as in refrigeration and cryogenic storage of biological samples.