What is Addendum?

Legal Definition
An addendum, in general, is an addition required to be made to a document by its author subsequent to its printing or publication. It comes from the Latin verbal phrase addendum est, being the gerundive form of the verb addo, addere, addidi, additum, "to give to, add to", meaning "(that which) must be added". Addenda is from the plural form addenda sunt, "(those things) which must be added". (See also memorandum, agenda, corrigenda.)
-- Wikipedia
Legal Definition
An attachment or exhibit to a document, such as a contract. For example, a contract to manufacture widgets may have an adendum listing the specifications for said widgets. As this modifies the original document, it should be signed or executed with the same formalities.
Legal Definition
A document attached to clarify or modify a part of a contract.