The combination of HSDPA and HSUPA technologies. These are enhancements to WCDMA / UMTS 3G technology that boost data speeds, among other benefits.
Newer versions are called HSPA+ and are capable of download speeds up to 21 or 42 Mbps. 42 Mbps speeds are achieved by basically combining two 21 Mbps networks (twice the radio spectrum is used for 42 vs. 21 Mbps networks.)