What involves singing or playing without accompaniment?

The concept of performing music without any instrumental accompaniment is known as "a cappella," which translates from Italian as "in the manner of the chapel." This style originated in the context of church music, where vocal performances were predominantly executed without instruments to emphasize the human voice's clarity and expression. A cappella performances can feature various vocal arrangements, including harmonies, and often allow for additional vocal techniques such as beatboxing or vocal improvisation to provide texture and depth.

In contrast, solo performance refers to a single musician performing alone, which can include instrumental or vocal music with or without accompaniment. Unison involves multiple voices singing the same melody together, which does not imply the absence of accompaniment, while chamber music generally involves small ensembles with instruments, and may or may not include vocals. A cappella specifically denotes a vocal performance free of any supporting instruments, making it the correct answer for this question.