The percentage of all television households in a survey area with one or more sets in use during a specific time period. The sum of the average ratings for a given time period will sometimes be higher than the HUT number because of households viewing multiple programs at the same time. If a household is watching two programs, it is counted toward each program rating but only once toward a HUT number.

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