The same as "Vertical Angles": the angles opposite each other when two lines cross. They are always equal.

In this example a° and b° are vertical angles.

Try moving the points below. Notice that the 4 angles are actually two pairs of vertical angles:

In this example a° and b° are vertical angles.

Try moving the points below. Notice that the 4 angles are actually two pairs of vertical angles:

See: Vertical Angles