Final answer:
A polygon is not an undefined term in Euclidean geometry because it can be defined using lines, which are themselves an undefined term.
Step-by-step explanation:
The concept not considered an undefined term in Euclidean geometry is B. Polygon.
In Euclidean geometry, undefined terms serve as the basic building blocks for defining all other shapes and concepts; they are accepted as intuitive concepts without formal definitions. Examples of undefined terms include point, line, and plane. These terms are assumed as self-evident and used to define more complex structures, like polygons. A polygon is actually a shape made up of a finite number of straight line segments connected to form a closed chain or circuit. Since a polygon can be defined using lines, and a line is an undefined term, the polygon itself is not an undefined term.