The bending of a wave from its original path, either because it’s passing from one medium to another or by changes in the physical properties of the medium, such as a temperature or wind gradient in the air. Refraction affects both the direction and speed of the wave.
The most common audio example of this phenomenon is the speed of a sound wave in air, which varies as the air temperature changes. Sound waves travel faster in warm air, which is less dense than cold air. For example, during the day the air is warmest right next to the ground and grows progressively cooler as altitude increases. In the case of a temperature inversion, such as when the sun goes down and the ground cools off quickly, the air above the ground remains warm. Since the temperature increases with height, the speed of sound also increases. The wave also changes direction and bends downward. This is why you can hear distant sounds unusually clearly around sunset.