Terminology
The term originated in the Perpetual Union of the Articles of Confederation and Perpetual Union. The Constitution of 1787 opens with, "We the People of the United States, in Order to form a more perfect Union...do ordain and establish this Constitution for the United States of America." Even before the war started, the phrase "preserve the Union" was commonplace and a "union of states" had been used to refer to the entire United States. Using the term "Union" to apply to the non-secessionist side carried a connotation of legitimacy as the continuation of the pre-existing political entity.
Read more about this topic: Union (American Civil War)