The name of the mecha in the Gundam anime science fiction series. It basically refers to any (mostly) humanoid giant robot (stands from about 15 to 20+ meters) that is piloted by a human soldier (aside from G Gundam, which uses the term Mobile Fighter).
As said above, most mobile suits are humanoid-shaped, though there are some which are not, such as the BuCUE and LaGOWE from Gundam SEED.
This fictional term was first mentioned and introduced in the very first episode of the original Gundam series, Mobile Suit Gundam from 1979.
Tomino Yoshiyuki, creator and writer of the series, explained that the concept of a manned robot was inspired by the super robot shows of the 70s that began with the Mazinger Z anime from 1972.
The name is also a tribute to the Mobile Infantry, a fictional military organization of space marines that uses power armor in the novel Starship Troopers from 1959.