Formal:
* Antonym: This is the most direct and technical term for a word or phrase with the opposite meaning.
* Converse: This term is often used in logic and mathematics to describe a relationship where the truth of one statement implies the truth of the other, but not vice versa.
* Inverse: Similar to converse, but often implies a stronger relationship of reversal.
Informal:
* Opposite: This is a more general term that can be used for any word or phrase that means the opposite.
* Counterpart: This term is often used to describe two things that are similar in some ways, but opposite in others.
* Flip side: This term is used to describe the opposite perspective or viewpoint.
Examples:
* Antonym of "hot": cold
* Converse of "If it's raining, then the ground is wet.": "If the ground is wet, then it's raining."
* Opposite of "happy": sad
* Counterpart of "good": bad
* Flip side of "being optimistic": being pessimistic
The best term to use will depend on the specific context and the level of formality you want to achieve.