A backup vocal part is an essential element in creating a well-rounded musical composition. The backup vocals provide an added layer of richness and depth to the overall sound of a song and can help emphasize key lyrics or hooks. There are several synonyms for the term 'backup vocal part' which include harmony vocals, backing vocals, supporting vocals, and ad-libs. Harmony vocals emphasize specific parts of the melody while supporting vocals reinforce lead vocal lines. Backing vocals can be used to create call and response sections in a song, while ad-libs are spontaneous vocalizations that can add a unique touch to a performance. Regardless of the term used, backup vocals are crucial in completing a polished musical composition.