For those interested in a definition of terms, David Uption provided a reference to an 1861 publication posted here. The title is Manual of instruction for the volunteers and militia of the Confederate States, by William Gilham.

http://books.google.com/books?id=TPwlU_S_02YC&pg=PA516&dq=affair+skirmish+combat+action+engagement+and+battle&hl=en&ei=SbqKTdyMMYfCsAOgw4yUCg&sa=X&oi=book_result&ct=book-thumbnail&resnum=7&ved=0CEkQ6wEwBg#v=onepage&q=affair%20skirmish%20combat%20action%20engagement%20and%20battle&f=false

George W. Cullum's Biographical Register of the Officers and Graduates of the U.S Military Academy at West Point, N. Y. (1868), includes more precise definitions. To his language the following phrase should be added, "If the smaller of the two forces is [unit size], then...."

With that additional condition, we can surmise the following from Cullum,

Army = battle
Corps = engagement
Division = combat
Brigade = action
Regiment = skirmish
Lesser unit = affair

Since at least two regiments and probably more were involved on the Confederate side, the fight at Bardstown on Oct. 4, 1862, comes close to qualifying as an action.