Final answer:
In mathematics, the line of symmetry or line of reflection is a straight line that reflects one half of a shape or object to create a mirror image. It is a crucial concept for understanding the properties of symmetrical shapes in geometry.
Step-by-step explanation:
When considering the concept of symmetry in mathematics, especially when referring to the line of symmetry or line of reflection, the correct form is a straight line. This line acts as a mirror, reflecting one half of the object or shape over to the other. A symmetrical object or shape would thus appear identical on both sides of this line. The line of symmetry can be represented in various contexts such as geometry, where it helps determine the properties of shapes and figures.