What is the term for a contract proposal that modifies the terms of an original offer?

The correct term for a contract proposal that modifies the terms of an original offer is a counteroffer. This occurs when one party responds to an initial offer with changes to the terms proposed. By doing this, the original offer is effectively rejected, and the new terms presented become the basis for negotiation. A counteroffer represents a willingness to negotiate further, indicating that the initial terms were not acceptable as proposed, but still aims toward a potential agreement.

In contrast, an addendum is an attachment that adds new information or terms to an existing contract without changing the overall agreement. An amendment consists of changes made to the existing terms within a contract but does not involve an entirely new proposal that rejects the previous terms. Termination signifies the ending of a contract altogether rather than proposing any modifications to its terms.