A technique that essentially combines two normally separate radio frequency carriers into one, thus providing double the bandwidth. In this context, a carrier is a small sub-band of a larger radio frequency band.
DC typically translates to double the potential data rate (speed) for end users, compared to the same data technology without DC. For example, HSPA+ tops out at a theoretical maximum of 21 Mbps, but can be extended to 42 Mbps with DC.
Because it requires double the radio spectrum, DC can only deployed in areas where a network operator owns enough radio spectrum to do so. If the network operator is operating multiple technologies in the same band (such as WCDMA and LTE) that may further restrict its ability to deploy DC.