The music that begins a theatrical performance is known as the

While "opener" might suggest the beginning of a performance, it is a more general term and does not specifically refer to the musical introduction. It could pertain to any form of opening act or scene in various types of performances, lacking the specific connotation associated with an overture.

A segue refers to a smooth transition from one piece of music or scene to another. Although it is a useful term in music and theater, it does not apply to the introductory music that starts a performance. A segue occurs after the performance has already begun, making it an irrelevant option in this context.

The point of attack is a dramatic term that signifies the moment a story begins to unfold in a narrative. It does not refer to music but rather to the structure of the story itself. Thus, it is not related to the introductory music aspect of a theatrical production.

A set piece refers to a significant scene or moment in a performance, often involving elaborate staging or choreography. While important in theatrical productions, it does not describe the introductory music that occurs at the start of a performance.