What do you call a piece of historical information from a person who was actually at the event?

A piece of historical information from a person who was actually at the event is called a "primary source." Primary sources include firsthand accounts such as letters, diaries, photographs, and interviews, providing direct evidence of the time and context in which an event occurred. These sources are invaluable for historians as they offer insights into the experiences and perspectives of those who witnessed the events firsthand.