A concert (as in a "concert tour") is a performance in which a musician or many musicians play music together to a group of listeners (called an audience). In the case of a concert of classical music there may be an orchestra. There is often an important visual element to the performance, such as with a rock concert. However, if acting out a story is an important part of the performance, then this is called "theatre" or "opera" but some concerts have dramatic, theatrical music, with lights and special effects. Informal names for a concert include "show" or sometimes, "gig" for musicians who play in lots of different concerts, with different groups. A "concert tour" is a set of concerts where the performers travel to different cities or towns.