Overture
In the 19th century many Romantic composers wrote concert overtures. These pieces did not belong to any opera or ballet, they were just written to be heard at concerts. They often had a descriptive title because they told some sort of story, e.g. Felix Mendelssohn (1809-1847) wrote an overture "Fingal's Cave" which describes the sea coming into the cave in the Inner Hebrides islands. Sometimes these descriptive pieces were much longer than an overture (which is usually just a few minutes), so they were called tone poems.