It's the wireless phone transmitter/receiver. The term refers to the nature of the mobile phone technology. The orginal old mobile phones had massive trasmitters and received on the base station and mobile phones. A new generation of mobile phone technology was designed around using smaller base stations several miles /kilmoters apart around the service area. These smaller base stations are tied back to the central phone switch site via trunk lines. These sites have a smaller area to provide coverage for. So the transmitters on both side could be smaller. The sites are commonly called "cell sites" or "cell towers" inthe U.S. Hence the term cellular (or cell)O i.e. cellular phone or cell phone. (Picture cells in living tissue to get the idea). In the EU, I think the term is "Base Transciever Station".