Baseband usually refers to the firmware running the CPU/chip dedicated to operating the radio(s), generally the WAN radio (LTE, 3G, etc.) for a cellular connect

Baseband usually refers to the firmware running the CPU/chip dedicated to operating the radio(s), generally the WAN radio (LTE, 3G, etc.) for a cellular connection, although some are integrated with other types (WiFi, Bluetooth, NFC, etc.).