The local temperature of a position indicates the value of playing there, relative to continuations either player has in that part of the board. It is a number calculable by endgame counting techniques, when those apply; and is then the same as its miai value. A local position can be called hot when its local temperature is high compared to the other positions on the board. A go position will be hot to the extent that one player or other makes a gain by playing there. To put it another way, a position is hot if plays there are big.
Imagist: Hot is used more in CGT discussions, while the term Urgent seems to be preferred for strategic discussion. However, the terms are at least partly interchangeable.
Bill: Urgent has long had a specific go meaning relating to local tactics, as in Urgent plays before big plays. Better avoid it as a general term. Of course, urgent plays are hot.