HSDPA (High-Speed Downlink Packet Access) - definition
An upgrade for UMTS networks that doubles network capacity and increases download data speeds by five times or more.
The service was initially deployed at 1.8 Mbps but upgrades to the networks and new user devices led to increased rates of 3.6 Mbps, followed by 7.2 Mbps and further down the road, 14.4Mbps and even 21Mbps.
HSDPA only handles the downlink while the uplink is handled by a related technology called HSUPA. The combination of both technologies is usually called HSPA.