What are opposite angles?

Opposite angles are angles across from each other in intersecting lines. For example, see the figure below:

a-> X <- b

Angle 'a' and angle 'b' are equal to each other. The two unnamed angles at the top and bottom of the figure are also equal.

Polygons

Opposite angles are across the axis of the shape from each other, as in a parallelogram (quadrilateral) which has two pairs of opposite angles.