Its a collective oscillation of the electron density in a metal. It's most prominent in Au, Ag, and Cu, possibly due to the low imaginary part of the dielectric constant which means that the oscillations are more pronounced and less dampened.
What excalibur described is any plasmon - the vibrational eigenmode of the entire electron gas about the positive lattice. A surface plasmon is specifically that mode which propagates along the surface of a metal or the interface between two.