A sector of a circle is bounded by two radius lines and a portion of the circumference known as a/an:

A sector of a circle is defined as the area enclosed by two radius lines that extend from the center of the circle to the circumference, along with the curved line connecting these two points on the circumference. This curved line is specifically known as an arc.

In geometric terms, an arc represents a portion of the circumference of the circle, which is the continuous curve that makes up the circle's boundary. The term "arc" accurately describes this part of the circle that comprises the outer edge of the sector, thus making it the correct choice in this context.

Other terms in the list have specific meanings unrelated to the concept of a sector. For instance, a chord refers to a straight line segment whose endpoints lie on the circumference of the circle, but it does not include the curved portion. A segment pertains to the area enclosed by a chord and the arc, but it does not refer to the arc itself. An ellipse is a different shape altogether and does not apply to the properties of a circle. Therefore, the term arc directly relates to the sector of a circle and defines the curved boundary, making it the most appropriate answer.