A chip in a smartphone or tablet that handles the cellular transmission. The baseband processor converts the data to a signal that can be used to modulate the carrier frequency for transmission and vice versa. That signal is handed off to the RF processor, which may also be part of the baseband chip. See baseband
, Passive Wi-Fi
Baseband Processor in a Tablet
The baseband circuitry in a 3G/4G tablet comprises about 5% of the total components. Without it, there would be no cellular connectivity.