A feature of LTE networks that allows two separate radio frequency bands to act as one, improving data speeds and/or network capacity.
The technology can be used to boost top theoretical LTE data download rates from 100 Mbps to 150 Mbps or more, although on many networks it will be used primarily to improve network efficiency and therefore capacity.
Carrier Aggregation is the most notable feature of LTE Category 4 and above, often referred to as LTE-Advanced. Other features of Category 4 (and above) also help boost network efficiency and reliability. Network operators can pick and choose which is these features to deploy, but Carrier Aggregation is the most common.