A sworn written statement.

"Written" matters because this is testimony put on paper, not casual talk, not a text thread, and not something a lawyer can clean up into evidence just by attaching it to motion papers. "Sworn" matters even more: the person signing is declaring the facts are true under oath, usually before a notary, and false statements can lead to perjury charges. "Statement" does not mean proof all by itself. People routinely overrate affidavits. A signed affidavit can support a motion, explain events, or preserve someone's account, but it is still only one person's version unless other evidence backs it up.

In a New York injury case, affidavits often show up early and often: from the injured person, a witness, a doctor, or an investigator. They can help establish notice, identify road or weather conditions, or oppose a weak defense story after a crash on the Long Island Expressway during a nor'easter or a winter pileup on I-90 near Buffalo. But an affidavit with guesses, hearsay, or obvious lawyer-made wording can fall apart fast.

New York also has a useful wrinkle. Under CPLR 2106, as amended in 2014, certain unsworn affirmations can substitute for affidavits, and later changes broadened who may use them. That catches people off guard. The label matters less than whether the statement is legally valid, fact-based, and admissible in the lawsuit.