Final answer:
A smaller specialized vocal ensemble is referred to as a choir, distinguishing it from larger choruses, individual virtuosos, and the style of singing known as a cappella.
Step-by-step explanation:
A smaller specialized vocal ensemble is known as a choir. This term is specifically used to denote a group of singers who perform together, typically with a structure involving various voice parts such as soprano, alto, tenor, and bass.
Choruses and choirs can sometimes be used interchangeably, but a chorus may imply a larger group, whereas a choir often refers to a smaller, more specialized group. A virtuoso is a term used to describe a highly skilled musician, not a group. A cappella refers to singing without instrumental accompaniment rather than the size or type of ensemble.