A longitudinal wave is a type of wave where the particles of the medium move parallel to the direction of the wave's travel. This means that as the wave moves forward, the particles compress and expand in the same direction. Common examples of longitudinal waves include sound waves, which travel through air, liquids, and solids. In a longitudinal wave, areas where particles are close together are called compressions, while areas where they are spread apart are known as rarefactions. The speed of a longitudinal wave can vary depending on the medium it travels through, such as air, water, or solid materials like steel.