The Northern states were collectively 'The Union' and the Southern states were collectively 'The Confederacy.' It's important to note however, the name of the United States was the official name of the so-called Union. Sometimes the US is also referred to as the United States of America.
The Union
union
The Union
Abraham Lincoln
The Union or the Yankees.
The Confederate States of America
Substitute ( :
the Confederate union
The Confederate States of America.
The South liked to name them after the nearest town. The North liked to name them after the nearest water-course - river, stream or creek.
The United States of America instead of the Confederate States or the North.
Contrabands were runaway slaves in the North.