Overtures!

[relm1]

Sometimes an overture is really a symphonic poem.  For example, Tchaikovsky's Romeo and Juliet Fantasy Overture.  I think most of the overtures Malcolm Arnold wrote were symphonic poems.  I guess there is an overlap. 

[(poco) Sforzando]

There are stand-alone concert overtures that can overlap with symphonic poems, and overtures to operas which may or may not be the same as preludes. Is the Meistersinger an overture or prelude? Wagner calls it a Vorspiel, same as Parsifal, which I could translate as Foreplay except that sounds a bit scatalogical and I'm sure Jo498 would correct my faulty German. But Wagner also calls the overture to the Flying Dutchman (which Tovey, Hurwitz, and others consider the best thing in the opera) an Ouverture. So there.

Lots of good examples above. Let me add the overtures to the Gilbert and Sullivan operettas, the best of which in my opinion are Iolanthe and The Yeomen of the Guard. And from American musical comedy, the tiny overture (scored for a huge orchestra of piano and harp) from Jones' and Schmidt's The Fantasticks.