A feature of 4G and 5G networks that allows two separate radio frequency bands to be used simultaneously and effectively act as one, improving data speeds and/or network capacity.
Carrier Aggregation was not part of 5G NR at launch, but is being added to 5G phones and networks in 2021, making 5G faster.
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.