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.