The full form of QSFP-DD is Quad Small Form Factor Pluggable Double Density. QSFP-DD is a high-speed, high-density data center interconnect module designed to support data rates of 200 gigabits per second (Gbps) or 400 Gbps. It is an evolution of the QSFP form factor, featuring a doubled number of electrical lanes and mechanical connectors to achieve higher data transfer rates. QSFP-DD is used for various applications, including Ethernet and InfiniBand connectivity in modern data center environments.