Traveller started out as a 'generic' science-fiction RPG, and steadily evolved a setting through the late '70s and '80s. The setting has come to dominate the system, with four different versions of the original system released, and four different releases that are adaptations of other systems to the Traveller universe. With the latest release, the name has come home again, in a way. Traveller is the name of Mongoose Publishing's SF RPG line, with the Traveller universe just one of the settings it supports.
I've heard some unsettling things about Mongoose making some pretty drastic changes to elements of the setting, but I haven't looked into it enough to be able to say what's going on for myself.
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