The term "copperhead" applies to a northerner who sympathized with the Southern cause during The Civil War. In recent years, the term has been used to describe Peace Democrats, but the two were not synonymous during the Civil War.
Generally, copperhead was a derogatory term that applied to radicals who took action, whether in the legislature or near a field of battle. People who agreed with the Southern manifesto were called "doughheads." There seems to be some regionality to the two terms. Copperheads were in the west while doughheads were in the east.
The most commonly noted copperhead was Clement Vallandigham, who strongly supported the South's claim to states rights, but was strongly opposed to slavery.