Continuous media is data where there is a timing relationship between source and destination. The most common examples of continuous media are audio and motion video. Continuous media can be real-time (interactive), where there is a "tight" timing relationship between source and sink, or streaming (playback), where the relationship is less strict.
CSS can be used in a variety of contexts, including print media and some CSS, in particular, that used for layout behaves differently depending on the context it is in.
Continuous Media, therefore, identifies a context where the content is not broken up, it flows continuously. Web content displayed on a screen is continuous media, as is spoken content.