Mounted artillery, which is generally attached to and manoeuvres with infantry, the cannoneers marching at the sides of their pieces, or, when necessary, mounting the ammunition chests.
Source: French, Barry & Hunt, The 1864 Field Artillery Tactics, p.1.

For those interested in other periods of warfare, an ACW mounted battery was conceptually similar to a wurst battery of Napoleonic times, the name being a reference to the sausage-like cushion covering the caisson [Wurst-Wagen] where Austrian horse artillerists sat. An interesting deduction is that, by Napoleonic standards, all the ACW field artillery would have been classified as horse artillery, or at least semi-horse.