In media player circles (i.e. WinAMP, XMMS, Sonique), a visualization is a plugin that displays graphics and other "Pretty Pictures" that usually have a pattern in relation to the audio being played.
Visualizations have previously followed the basic lines of the classic oscilloscope look, but have recently begun to embrace 3D Accelerators to to some amazing 3D audio representations.
A lot of graphics coders who were too young to get in on the Demo Scene (when it was 'the big thing') have embraced the visualization world (and to a lesser extent, the Skin world) -- it is certainly one of the best places to flex your graphics coding skills.