A disturbance that moves through space or matter, often transferring energy in the process.
It can be in many forms, including sound waves in air, ripples on water, or electromagnetic waves like light.
It changes around some central value:
• either due to a "restoring force" like the tension in a spring, or
• for light waves the change in an electric field generates a magnetic field, and vice versa, and they dance together like that even through a vacuum.
See: Wavelength