In a contract, it means the terms by which the contract can be broken

The Contract Clause of the United States Constitution covers contract law. The clause was created to keep states from using "private relief" to allow certain individuals an escape from their financial obligations. The Contract Clause prevents states from enacting laws that impair legal contracts.

The Severance Clause, also known as a Severability Clause, is a legal provision that may be included in a contract or legislation that states that if part or parts of the contract or legislation is determined to be invalid, unenforceable or unconstitutional that the remainder of the contract or legislation is still valid or in effect. If a contract or legislation does not include a Severability Clause and any part of is ruled to be illegal or unenforceable then the entire contract or legislation is voided.

list the characteristics of a simple contract

You can only back out of a signed contract, if the contract states there is a backout clause. If there is no clause stating you may back out of the deal within 24 hour, you are liable for the contract signed.