"Art music" isn't a new term, it's been around at least since the early 20th century and has been very common in musicology circles at least since the 1960s. The term "classical music" has long been considered problematic because it can be a little bit confusing -- it can refer to the entire genre, or to music in the style associated with a very specific period. Quite a few musicians and scholars have used the terms "art music" or "Western art music" to refer to the entire (lowercase c) classical genre, in order to avoid confusion. Some people also prefer to use the term "art music" because it can encompass a lot more than just music in the European classical tradition.