Quadrature Phase Shift Keying.

QPSK is a digital frequency modulation technique used for sending data over  networks of several media, including 802.11 wireless.

Doubling the data rate available in [BPSK], via improved efficiency in the use of the radio bandwidth, QPSK is used in [802.11b] networks of 2Mpbs or greater.

* (http://www.pulsewan.com/data101/802_11_b_basics.htm 802.11b Basics)

See also [PHY].