What defines opposite angles?

Opposite angles, also known as vertical angles, are defined as the angles formed when two lines intersect. They share a common vertex and are located opposite each other. This definition highlights the important characteristics of opposite angles: they are always congruent, meaning they have the same measure. The presence of the intersecting lines and the shared vertex are essential to their formation. This is crucial in geometry as it establishes relationships between angles that can simplify calculations or prove relationships in various problems.

The other choices do not correctly define opposite angles. For instance, angles that sum to 90 degrees refer to complementary angles, not opposite angles. While opposite angles are indeed always equal, the focus of the definition is more accurately on their relation to intersecting lines and sharing a vertex. Lastly, stating that they do not meet at any point contradicts their very definition since they are formed at the intersection of two lines.