Use the word minus to mean "less" or "with the subtraction of." When it's minus fifteen degrees outside, it's fifteen below zero — or fifteen degrees less than zero.
Whenever you talk about negative numbers, whether they relate to temperature or your bank account, the adjective minus always applies. If you have minus fifty dollars in the bank, you're below zero on the money scale. You can also use minus as noun, to mean "subtraction," or a subtraction mark: "Put a minus in front of that three before you add the columns." Minus means "less" in Latin, and it's closely related to minor, or "smaller."