Final answer:
The most important characteristic distinguishing a primary historical source is that the author experienced the event firsthand.
Step-by-step explanation:
The most important characteristic distinguishing a primary historical source from secondary or tertiary sources is that the author experienced the event firsthand.
Primary sources are original materials that provide direct evidence or firsthand information about a historical event or topic. They can be diaries, letters, photographs, speeches, artifacts, or eyewitness accounts.
For example, a primary historical source could be a diary written by a soldier during a war, while a secondary source could be a history textbook that analyzes and interprets various primary sources.
Learn more about Primary vs. secondary historical sources