UAPSD is an acronym for Unscheduled Automatic Power Save Delivery, a feature of Wi-Fi

Wi-Fi

Wi-Fi or Wifi, is a mechanism for wirelessly connecting electronic devices. A device enabled with Wi-Fi, such as a personal computer, video game console, smartphone, or digital audio player, can connect to the Internet via a wireless network access point. An access point has a range of about 20...

 devices that allows them to save power.
UAPSD is also known as WMM power save

Practical Uses

The Nokia 6086 GSM/UMA cellular phone, one of the two phones currently offered by T-Mobile USA

T-Mobile USA

T-Mobile USA, Inc. is an American mobile-network operator, headquartered in Bellevue, Washington, that provides wireless voice, messaging and data services in the United States, Puerto Rico and the U.S. Virgin Islands. The company is the fourth-largest wireless carrier in the U.S. market with 33.73...

 for the HotSpot@Home product, uses UAPSD to extend handset battery life from approximately one day to two or three days, under normal phone usage conditions.