Traveller is a system for playing generic space-opera themed science fiction adventures.
Originally Traveller was intended to be a system for playing generic space opera-themed science fiction adventures in much the same sense that Dungeons & Dragons was a system intended for generic fantasy adventures. Marc Miller, one of the original designers of the Traveller RPG for Game Designer's Workshop, said that the idea for creating Traveller came about when he said "I want to do Dungeons & Dragons in space." Most published supplements for Traveller deal in some way with a default setting called the "Third Imperium", (sometimes referred to as the Official Traveller Universe (OTU)), but the main rules are generic enough so that a campaign can be played in any setting the referee chooses.
Description taken from Wikipedia.