A transciever device which connects a wireless Ethernet network (typically IEEE 802.11b, aka AirPort) to a wired network. If your wireless-equipped machine (e.g. your laptop) is within range of a base station, you're on the net (if the base station will talk to you, that is).

(P.S. I can't resist pointing out that I'm typing this on a PowerBook connected to the net via a wireless network and just such a base station...)