The term is almost exclusively applied to works in the European classical music tradition, particularly those from the Romantic music period of the 19th century, during which the concept was popular, but pieces which fit the description have long been a part of music. The term is usually reserved for purely instrumental works (pieces without singers and lyrics), and not used, for example for opera or lieder. Single-movement orchestral pieces of program music are often called symphonic poems. Absolute music, in contrast, is intended to be appreciated without any particular reference to the outside world.