Well technically a bus is a group of wires shared amongst several devices. Each device has to wait its turn to talk/listen.

PCI, AGP, ISA, IDE are busses.

PCI express, USB, SATA are not busses but rather point-to-point protocols.

The usage of the term has changed a bit. From WP:

"Early computer buses were parallel electrical wires with multiple hardware connections, but the term is now used for any physical arrangement that provides the same logical function as a parallel electrical bus. Modern computer buses can use both parallel and bit serial connections, and can be wired in either a multidrop (electrical parallel) or daisy chain topology, or connected by switched hubs, as in the case of USB. "

(The B in USB stands for Bus)