An overture is an instrumental work for orchestra which was originally always composed as the introduction or prelude to an opera, ballet, or stage play. Well-known examples a
…bound by Beethoven, Rossini, Verdi, and Wagner, which are often played in the concert hall independent of the larger works to which they refer. .
The "concert overture" is likewise a single-movement symphonic piece, which usually has some programmatic or extra-musical point of reference, but does not belong to an opera or other larger work. Mendelssohn's "Hebrides" and Brahms' "Tragic Overture" are examples which may be vaguely pictorial or loosely conceptual, respectively. Such woks are closely related to the "symphonic poem", which purports to depict a story, scene, or idea in music, but are typically too elaborately wrought to be classed as concert overtures, as in the works by Lizst and especially Richard Strauss. Sibelius perhaps arrived at a happy medium. .
There is also a less frequently encountered category of concert overture that does not invoke any descriptive title at all, such as Bruckner's Overture in g-minor. Such works may impress one as being the first movement of an incomplete symphony. .
A further use of the term is the baroque French "ouverture", which did originally precede stage productions, but acquired independed concert-music status as the form was elaborated into a large multi-tempo introduction - the "ouverture" proper - followed by a suite of assorted dance movements. The entire composition would be called a suite or an ouverture depending on local usage, the German Telemann prefering the French nomenclature, while Bach had his "English" and "French" suites for keyboard, as well as a "Partita" (the Italian equivalent) specifically titled "Ouverture in the French Style", and which stands apart from the set of six partitas proper. ( Full Answer )