A declarant is a person who makes a statement they allege to be true. Typically, such statements are written and signed. A declarant is sometimes referred to as an affiant if the declarant's statement takes the form of a sworn affidavit.
Under the Rule 801 of the Federal Rules of Evidence, a “declarant” is a person who makes a statement. That statement need not be written. Note that the declarant must be a human; a computer is not a “person” and thus cannot be a declarant. Who is and who is not a declarant is an important distinction because evidence at trial may be inadmissible as hearsay based on this distinction. For instance, the court in Gayle v. State found that a report that contained text messages between the defendant and another person was not hearsay because the report was generated by a machine and thus not a “statement” made by a “declarant.”
See e.g., People v. Clay, 88 A.D.3d 14, 2011 N.Y. Slip Op. 5729, 926 N.Y.S.2d 598 (N.Y. App. Div. 2011)
[Last updated in September of 2022 by the Wex Definitions Team]