A wave is any distance that transmits energy through matter or empty space. Energy can be carried away from its source by a wave. Waves from the rocks splash carry energy away from the splash. However, the material through which the wave travel does not move with the energy. As a wave travels, it does not work on everything on its path. But most waves transfer energy by the vibration of particles in a medium. Therefor, a medium is a substance through which a wave can travel. A medium can be a solid, liquid, or a gas. There is also a transverse wave and a longitudinal wave. A transverse wave is a type of wave that the particles  vibrate in an up and down motion. A longitudinal wave is when the particles of the medium vibrate back and forth along the path that the moves. A part of a longitudinal wave where the wave the particles are crowded together are called a compression.  A part where the particles are spread apart is a rarefactions. Finally, a crests is up and trough is down.