In physics, X-waves are localized solutions of the wave equation that travel at a constant velocity in a given direction.
X-waves can be sound, electromagnetic, or gravitational waves.
Ideal X-waves carry infinite energy, but finite-energy realizations have been observed in various frameworks.
X-wave pulses can have superluminal phase and group velocity.
[1] In optics, X-waves solution have been reported within a quantum mechanical formulation.