An addendum to a contract is an addition or change to the terms of an existing contract that is signed by all parties to the original agreement, according to The Law Dictionary. It describes any changes or modifications to a contract, while leaving it fully enforceable.
A contract addendum must be carefully drafted because contract law requires the parties to abide by all terms of the document as it is written, notes The Law Dictionary. The parties must be careful to reference the original agreement and only change the terms that they all agree to by listing the terms and parts of the original contract that it modifies. The ultimate goal is to ensure that there are no unintended results that compromise the full force and effect of the contract.