Internal wave refers to the wave motion that occurs within a fluid body, typically within the ocean or atmosphere. Synonyms for the word "internal wave" include undular bore, internal swell, and internal tide. An undular bore is a wave front that propagates through a fluid body with a constant height and a sharp leading edge. Internal swells are waves that undergo refraction and reflection within the stratified layers of the ocean, and are driven by the wind. Finally, internal tides are the regular, large-amplitude waves generated by the gravitational attraction of the Sun and Moon on oceanic water. These synonyms for the word "internal wave" highlight the diverse forms that wave motion can take within fluid environments.