The Mesa is a grassy and moderately forested plateau around the south central region of the Island.
Roger Linus suggested to his son Ben that they celebrate Ben's birthday by drinking beers on "the Mesa." Ben used this opportunity to kill his father as part of the Purge. ("The Man Behind the Curtain")
As seen in "Pilot, Part 1"
Sayid, Locke, Desmond, Nikki, and Paulo cross the Mesa on their way to the Pearl. ("The Cost of Living")
The survivors climb the Mesa on Day 2 to find a signal for the transceiver. ("Pilot, Part 2") Later, Hurley built a golf course on the Mesa. ("Solitary") The capsule dump is located near the Mesa. ("Live Together, Die Alone, Part 1") Nikki, Paulo, Locke, Sayid, and Desmond crossed the Mesa while traveling to the Pearl, ("The Cost of Living") and Jack, Sawyer, and Locke crossed it when searching for Michael. ("The Hunting Party") The Mesa was also the location where Hurley, Charlie, Sawyer, and Jin took the DHARMA van for a ride. ("Tricia Tanaka Is Dead") Locke, Sawyer, Juliet, Miles, Charlotte, and Daniel cross the Mesa on their way to the Orchid. ("The Little Prince")
The Mesa after a time shift ("Jughead")
The specific term "Mesa" was used for this location by Roger on the day of the Purge:
ROGER: Well, I'll tell you what. All we gotta do this morning is run this stuff out to Pearl station. Why don't we go up to the Mesa, drink some beers? Have some... I dunno, father and son time.
- Lost Virtual Tour - filming location of the Mesa