What term describes an officer's firsthand witnessing of a crime?

The term that describes an officer's firsthand witnessing of a crime is "direct evidence." This type of evidence is based on personal observation or experience regarding an event or fact. In a legal context, direct evidence can include eyewitness accounts, where the observer has seen the events transpire.

Direct evidence stands in contrast to circumstantial evidence, which relies on inference rather than direct observation, and to other types of testimonies that do not come from firsthand experience. This distinction is critical in law enforcement and legal proceedings, as direct evidence often carries more weight in establishing the truth of the matter at hand.