The correct answer is: a PRELUDE
A prelude is a musical composition that is usually brief. It is characterized by being played as an introduction to a larger musical piece. It comes from the latin "praeludere" which means "to play before". In the beginning it was played as a necessity as musicians needed to warm up and test their tunning and the acoustics of the place, so they improvised these preludes before starting the main pieces. As time passed by, this tradition became part of the ritual of music and was included in the composition process.
