according to Encarta, this is the definition of disco:
1. club or party with dancing: a club or party where people dance to recorded pop music, often introduced by a DJ
2. steady-beat pop music for dancing: a style of pop music with a steady pronounced beat, popular in the 1970s for dancing. It developed from soul music, in response to the growing popularity of discos.
3. dance done to disco music: popular dancing with hips and arms moving to the repetitive beat of disco music

now the part that made me smile,
here is the definition of disco for french encarta:

1. musique de danse rapide et répétitive apparue en France, puis aux États-Unis, à la fin des années soixante-dix
(adorer le disco)

translation: Repetitive rapid beat dance music introduced in France then in the US in late 70's.