What do you call two adjacent angles that form a straight line?

Two adjacent angles that form a straight line are known as a linear pair. This term specifically refers to the situation where the angles share a common vertex and a common side, and the other sides of the angles form a straight line. The straight line indicates that the angles together measure 180 degrees, which is a defining characteristic of supplementary angles.

While supplementary angles refer to any two angles that add up to 180 degrees, a linear pair specifically describes the geometric relationship where the angles are adjacent and their non-common sides form a straight line. This makes "linear pair" the precise term for the scenario presented in the question.