Isengard is the city's name in the language of Rohan. It is a translation of the Sindarin name Angrenost meaning "Iron Fortress".
Isengard (or better, the tower of Orthanc) possibly is one of the places which gave name to the second volume of The Lord of the Rings, which is called The Two Towers. However, there are other candidates, and Tolkien states in one of his letters that he deliberately left the title ambigious.
Isengard is also the name of two Tolkien Metal bands, see Isengard (band).