What is the legal term for a person who testifies in court about what they have seen or heard?

The legal term for a person who testifies in court about what they have seen or heard is "witness." A witness is someone who provides evidence that can help establish facts in a case, whether they are sharing firsthand knowledge of an event, experiences related to the case, or any other relevant information. Witnesses can be called by either the prosecution or the defense in criminal trials, or by the plaintiff or defendant in civil cases, and their testimonies are vital to the judicial process, as they contribute to the establishment of truth in the court proceedings.

In contrast, the other terms listed refer to roles that do not involve giving testimony based on personal observation or experience. The defendant is the individual accused of a crime or being sued, the plaintiff is the party who brings a lawsuit to court, and the judge is the official who presides over the courtroom proceedings and makes legal decisions based on the evidence presented. Therefore, the distinction lies in the specific role and function of the witness in delivering evidence compared to the other roles mentioned.