an expert who observes and comments on something
"The sports commentator broke down every play as it happened, offering his expert analysis of the team's strategy."
a writer who reports and analyzes events of the day
"The nightly news program relied on their veteran commentator to break down the complex economic shifts affecting global markets."
A person who comments; especially someone who is paid to give his/her opinions in the media about current affairs, sports, etc.
"The sports commentator provided a lively analysis of the game while explaining the rules to viewers who were unfamiliar with them."