The gradual joining of Earth's nations under one world government after the Third World War, a process usually treated as complete by 2150 when the last independent states joined.
United Earth grew out of the recovery that followed the Third World War and first contact with the Vulcans. Over the next decades war, hunger and poverty were largely eliminated and the old nations came together under a single planetary government.
The date of 2150 comes from The Next Generation episode Attached, in which Beverly Crusher discusses the unification of Earth and imagines a single holdout nation as late as that year. Enterprise shows the result: a United Earth government with its own Starfleet, ministers and embassy network.
United Earth went on to host the Coalition talks of 2155 and became a founding member of the United Federation of Planets in 2161.