Trammel is a facet, or "map," within UO.
Trammel is a mirror of Felucca, UO's original map, except that non-consensual PvP is not allowed. It has also not been allowed in UO's subsequent facets. Facets whose rules do not allow for non-consensual PvP have come to be called "Trammel Rules Facets." (Example: "Ilshenar is a Trammel Rules Set Facet.")
The fiction of Trammel is that it was created by Nystul, the court mage of Lord British, in order so that Britannians could have a place to live that was free from the ravages of Minax, who was, in addition to the PKer/murderer problem, attacking Britannia at the time (especially the City of Trinsic).
After Trammel, the Factions fought for control of Felucca, whereas Trammel was ruled by Lord British and later the Britannian Council as well as the various role-played High Councils that exist on various shards.
Due to its lack of non-consensual PvP, Trammel rapidly became a very popular facet. Players who wished to focus on PvM, role-playing, and other activities could play without fear of constantly being attacked.