In basic terms the opposite of heat, or its absence. It's a strange term if you think about it. When describing 'brightness' you have light and then dark, being the absence of light. With light you do not have a term for 'less light' or a term for 'more dark'. In heating, ventilation and mechanical engineering if you apply energy to a substance or system you are adding 'heat', likewise if you remove energy you can be said to be 'adding' coolth.
In basic terms the opposite of heat, or its absence. It's a strange term if you think about it. When describing 'brightness' you have light and then dark, being the absence of light. With light you do not have a term for 'less light' or a term for 'more dark'. In heating, ventilation and mechanical engineering if you apply energy to a substance or system you are adding 'heat', likewise if you remove energy you can be said to be 'adding' coolth.