From Niall Kearney:

Reactivity measures the popularlity of new tracks and new releases. Reactivity is a feature that is becoming popular with the music industry. Producers, labels, and artists become increasingly aware of the popularity of their song. Big Champagne is an example of software that tracks downloads on sites. This software tracks information from sites such as Kazaa and iTunes. It calculates the number of times a song has been downloaded, how frequently the song has been played in the same geographical location and figures a measure of reactivity from the result.