This video lesson defines compressible waves as disturbances in a flow field. Such waves come in two varieties: A shock wave is a disturbance that propagates through the gas faster than the local speed of sound; an expansion wave occurs when supersonic flow meets a suddenly expanding area. This lesson is part of the Ansys Innovation Course: Shock–Expansion Theory. To access this and all of our free, online courses — featuring additional videos, quizzes and handouts — visit Ansys Innovation Courses at https://www.ansys.com/courses.