Riddermark means "land of the knights", derived from the Old English compound Riddena-mearc ("riders' mark" or "The Territory of the Knights"). Mark here is used in the sense of "borderland, especially one serving as a defence of the inner lands of a realm". The original untranslated Rohirric term of the country was Lōgrad.
Sindarin
rohan
place name. Riddermark, (lit.) Horse-country
The home of the Rohirrim, translated “Riddermark” (LotR/262) or more literally “Horse-country” (RC/241). It is a combination of roch “horse” and -(i)an “-land”, with the [[s|[x] (“ch”) softening to [h] in Gondorian pronunciation]] (LotR/1113).
Conceptual Development: In Lord of the Rings drafts from the 1940s, this land was first named N. Thanador of unclear meaning, which underwent several revisions (Ulthanador, Borthendor, Orothan[ador]) before Tolkien settled on N. Rohan (RS/434). At this earlier stage, Tolkien posited that this name developed from (ON.?) Rochan(dor); Tolkien coined the archaic form †Rochand later, while working on the drafts of the Lord of the Rings appendices (PM/53).