The ratio of cumulative to average quarter-hour audiences. Turnover is a valuable index of the consistency of an audience. If turnover equals 1.0, all the station’s audience for a given time period or daypart is tuned in during the average quarter hour of the daypart. If turnover equals 2.0, there are twice as many persons in the cume audience of the daypart as in the quarter hour.

Source:
http://www.nielsenmedia.com/glossary/
http://www.arbitron.com/downloads/terms_brochure.pdf