A subpoena is a legal document issued by a court that commands an individual to appear in court to provide testimony or produce evidence. It is crucial in legal proceedings to ensure the presence of witnesses or relevant information. Unlike medical certificates or prescriptions, a subpoena specifically serves to compel participation in a judicial process. Failure to comply with a subpoena can result in legal penalties. This is why a subpoena is best described as a court order to appear.

Reference: Medical Jurisprudence, Snell, 13th Edition.