Cool, like cold, is a word we use to describe a feeling. It is not a scientific way of talking about temperature, but a word used to talk about something. For example, we might say "I am glad that drink has cooled down" even though the drink might still be "hot". Or we might say "It is cool today" meaning "below what we think is about average".

Cool often means "Not too bad" and is one notch up from "cold" which usually means "uncomfortable".