The Union was the central government of the United States during the American Civil War. Its civilian and military forces resisted the Confederacy's attempt to secede following the 1860 election of Abraham Lincoln as president of the United States. Lincoln's administration asserted the permanency of the federal government and the continuity of the United States Constitution.

Nineteenth-century Americans commonly used the term Union to mean either the federal government of the United States or the unity of the states within the federal constitutional framework. The Union can also refer to the people or territory of the states that remained loyal to the national government during the war. The loyal states are also known as the North, although four southern border states and the future state of West Virginia remained loyal to the Union, and Black Southerners and many Southern Unionists opposed secession and supported the Union war effort.

The Northeast and Midwest provided the industrial resources for a mechanized war, producing large quantities of munitions and supplies and financing the war. They provided soldiers, food, horses, financial support, and training camps. Army hospitals were also set up across the Union. Most Northern states had Republican governors who supported the war effort and suppressed anti-war subversion. The Democratic Party supported the war at the beginning in 1861, but by 1862 it split into the War Democrats and the anti-war element known as Peace Democrats, led by the "Copperheads". The Democrats made major electoral gains in 1862 in state elections, most notably in New York. They lost ground in 1863, especially in Ohio. In 1864, the Republicans and War Democrats joined to campaign under the National Union Party banner, which also attracted most soldiers, and scored a landslide victory for Lincoln and his entire ticket against Democratic candidate George B. McClellan.

The war years were quite prosperous except where serious fighting and guerrilla warfare ravaged the countryside. Almost all military actions took place in the Confederacy. Prosperity was stimulated by heavy government spending and the creation of an entirely new national banking system. The Union states invested a great deal of money and effort in organizing psychological and social support for soldiers' wives, widows, and orphans, and for the soldiers themselves. Most soldiers were volunteers, although after 1862 many volunteered in order to escape the draft and to take advantage of generous cash bounties offered by states and localities. Draft resistance was notable in some larger cities, especially in parts of New York City, with its massive anti-draft riots of July 1863 and in some remote districts such as the Coal Region of Northeastern Pennsylvania.

In the context of the American Civil War, the Union, or the United States, is sometimes referred to as "the North", both then and now, while the Confederacy was often called "the South".

The term "Union" occurs in the first governing document of the United States, the Articles of Confederation and Perpetual Union. The subsequent Constitution of 1787 was issued and ratified in the name not of the states, but of "We the People of the United States, in order to form a more perfect Union ..." Union, for the United States of America, is then repeated in such clauses as the Admission to the Union clause in Article IV, Section 3. Even before the Civil War began the phrase "preserve the Union" was commonplace, and a "union of states" had been used to refer to the entire United States of America. 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. Before the American Civil War, the United States was known as the "United States' federal union", a union of states controlled by the federal government in Washington, D.C. This was opposite to the CSA's first government, a confederation of independent states, functioning similarly to the European Union. Confederates generally saw the Union as being opposed to slavery, occasionally referring to them as abolitionists, in reference to the U.S. Navy as the "abolition fleet" and the U.S. Army as "abolition forces".

In 2015, historian Michael Landis called for an end to the use of the term "Union", writing "The employment of 'Union' instead of 'United States,' implicitly supports the Confederate view of secession wherein the nation of the United States collapsed.... In reality, however, the United States never ceased to exist.... The dichotomy of 'Union v. Confederacy' lends credibility to the Confederate experiment and undermines the legitimacy of the United States as a political entity." In 2021, the Army University Press noted that it was replacing usages of the word "Union" with "Federal Government" or "U.S. Government". The Army University Press stated this was "more historically accurate" as "the term 'Union' always referred to all the states together."

Compared to the Confederacy, the loyal states were relatively more industrialized and urbanized and possessed more advanced commercial, transportation and financial systems. Additionally, the Union states had a manpower advantage of five to two at the start of the war.