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An Alfvén wave is one of the numerous types of plasma waves. Alfvén waves are transverse waves along magnetic field lines within the plasma. Ions drawn away from alignment along field lines are restored by magnetic force, such a disturbance traveling as a wave along the field line, remarkably similar to a taut string's traveling wave. The terms Alfvén velocity and Alfvén speed refer respectively to the velocity and speed of such waves, the latter which takes a value that can be computed from the plasma density and magnetic flux density (B). The terms sub-Alfvén speed and super-Alfvén speed are used to indicate speeds slower or faster than Alfvén speed. An Alfvén Mach number (aka Alfvén Number) is a Mach number specific to the speed of Alfvén waves, i.e., the plasma flow speed divided by the Alfvén speed. Within a plasma, such as the solar corona, the Alfvén point is a point (e.g., on a line outward from the Sun) where the Alfvén number is 1, i.e., the flow speed matches the Alfvén speed.
Much of the universe is plasma, making these concepts relevant, but the terms are commonly seen regarding the solar corona, an area of research-interest for being incompletely understood, and because of the corona's role regarding space weather.