The OVERTURE is the orchestral introduction to an opera.
The opera 'William Tell' was composed by Rossini. That included the overture, of course.
According to Wikipedia: "The overture to the opera William Tell, with its high-energy finale, is a very familiar work composed by Gioachino Rossini in 1829."
An instrumental introduction to an large work, like an oratorio or opera.
One of his greatest contributions was the verismo opera; an opera which has no overture, but rather the drama and staging begins immediately.