Define "a cappella".

A cappella refers to vocal music that is performed without any instrumental accompaniment. In this style, singers rely solely on their voices to create harmonies, melodies, and rhythms, making it a unique and often challenging form of musical expression. The use of vocal techniques, such as beatboxing or vocal percussion, can enhance the performance, but the essential characteristic is the absence of instruments. This form of singing has a rich history across various cultures and musical genres, showcasing the versatility and power of the human voice.