What term describes a person who testifies under oath at a trial?

The term that describes a person who testifies under oath at a trial is "witness." A witness is an individual who provides evidence, either by speaking about what they have seen, heard, or experienced, or by presenting other types of information pertinent to the case. This testimony is given in a court setting, where the witness is required to take an oath to tell the truth.

The other terms mentioned are specific to different contexts. A defendant refers to the individual accused of a crime or being sued in a civil case, and therefore does not primarily serve the role of providing testimony at trial. An affiant is someone who has made a sworn statement, typically in a written document or affidavit, but does not necessarily testify in person during a trial. A deponent is someone who gives testimony under oath, usually in a deposition, which is a pre-trial procedure to gather evidence but does not take place in the courtroom setting. Thus, the term "witness" is the most accurate description for someone testifying in court.