Definition of 'Addendum'
An attachment to an a document such as an insurance policy that changes the original policy conditions, either to include or exclude coverage. The addendum becomes part of the legal contract.
Investopedia explains 'Addendum'
In insurance, endorsement is an addendum attached to the original policy that alters or replaces the previous provisions.
In real estate purchase agreements, an addendum is a document containing any changes or modifications negotiated in the original lease. Usually, an addendum is attached to the signed lease, as a part of the lease, and describes financing terms and property inspection requirements.
For contracts, if the addendum alters a contract, which has already been signed by all parties, it is known as an amendment.