An affidavit is a written statement voluntarily made by an affiant or deponent under an oath or affirmation which is administered by a person who is authorized to do so by law. Such a statement is witnessed as to the authenticity of the affiant's signature by a taker of oaths, such as a notary public or commissioner of oaths. An affidavit is a type of verified statement or showing, or in other words, it contains a verification, which means that it is made under oath on penalty of perjury, and this serves as evidence for its veracity and is required in court proceedings.

An affidavit is typically defined as a written declaration or statement that is sworn or affirmed before a person who has authority to administer an oath. There is no general defined form for an affidavit, although for some proceedings an affidavit must satisfy legal or statutory requirements in order to be considered. An affidavit may include a commencement which identifies the affiant, an attestation clause, usually a jurat at the end certifying that the affiant made the statement under oath on the specified date.

In some cases, an introductory clause, called a preamble, is added attesting that the affiant personally appeared before the authenticating authority. An affidavit may also recite that the statement it records was made under penalty of perjury. An affidavit that is prepared for use within the context of litigation may also include a caption that identifies the venue and parties to the relevant judicial proceedings.