Olympus Mons is a very large shield volcano on the planet Mars. By one measure, it has a height of nearly 22 km Olympus Mons stands about two and a half times as tall as Mount Everest's height above sea level. It is the youngest of the large volcanoes on Mars, having formed during Mars's Amazonian Period. (wikipedia entry)
A mons is a mountain. On Mars they are named after the nearest named albedo feature on the Schiaparelli or Antoniadi maps.