The distance from the acoustic centre of the source to the point at which the intensity of the direct sound equals that of the reverberant field . References: Critical Distance SFU.ca
In audio physics, the distance at which the sound pressure level of the direct and the reverberant field are equal. In other words, the point in space where the echo from a sound source is at the same volume (amplitude) as the source. This distance is dependent on the geometry in which the sound waves propagate, as well as the dimensions and shape of the sound source. Wikipedia
See also: L1® vs B1 Bass Module Fall Off with Distance ⇐ Hilmar-at-Bose talks about Critical Distance and the L1®.
See also: Throw Over Distance ⇐ Comparing the drop-off in volume over distance