This reminds me of the accepted terminology of "alloy". Technically steel is an alloy as is titanium but in the cycling world, most people are referring to an aluminum alloy when they say "alloy". Carbon fibre is a composite. However, not all composites are carbon fibre based. Glass and kevlar fibres amongst others can also be used in composites. A composite is simply a material made using dissimilar categories of other materials. In the case of carbon fibre, it would be a plastic resin combined with carbon filiments. The term composite in the cycling world has throughout history meant different things. Sometimes the term refers to the construction methods rather than just the materials used. For instance, a while back aluminum tubes glued to steel lugs were considered composite bikes. Later on carbon fibre tubes glued to aluminum lugs were called composites.