The main fighting weapon in the Gundam universe - a series of Japanese TV shows and movies.
Though Mobile Suit Gundam wasn't the first of the Japanese Giant Robot genre, it was perhaps the first to attempt to be rational about it. In the series, there were somewhat reasonable reasons why people fought in giant humanoid robots - the most important of which was the development of a new,efficient, compact power source which emitted Minovsky particles, which jammed radar. Hence, missiles didn't work properly. In order to exploit this, the space based Zeon forces developed the MS-05 Zaku I with the Zeonic Company. This was the first mobile suit. At the beginning of the One Year War between the space-based Zeon forces and the Earth-based Federation, the Zeon forces annhilated the Federation using the vast superiority of the MS-06 Zaku II compared to conventional tanks, fighters, and capital ships. This superiority lasted until the Federation developed mobile suits of their own - including the legendary RX-78-2 Gundam, but mainly the mass-produced beam-weapon equipped RX-79 GM.
Mobile suits are roughly humanoid in shape, usually about 17 meters tall. The cockpit is usually found in the abdomen area. Though some have built in weaponry (MS-07 Gouf), they usually come with external weapons, like machine guns, beam guns, giant-superheated tomahawks, and beam sabers. Federation and Zeon mobile suits have a certain look to them that make them easy to tell apart. Federation mobile suit construction typicially begin with an endoskeleton on which components and armor are hung. This gives them a somewhat slender build. Zeon mobile suits typically start with a thick exoskelton in which they stuff the mechanics. This gives them somewhat more bulbous shapes.
Non-humanoid fighting units are called mobile armors, and usually are bigger and more powerful.