“What is the proper spelling: ‘Compact Disk,’ or ‘Compact Disc?’ And who decided on it?”
This is one I know everyone is going to have an opinion about, so the inSync mailbox is ready and waiting for you.
For all practical purposes, “disk” and “disc” are interchangeable words. That is, their definitions are the same. However, the context in which the term is to be applied often suggests which form is used. Personally, I will usually use “disc” when describing optical media, and “disk” when describing magnetic media. I’ve never seen anyone formally set that out as a rule, but I’ve noticed over the years that this is how they are most often used. You’ll rarely see the phrase “compact disk,” for example. When talking about phonograph records the term disc is most often used, as in disc jockey. The Latin roots come from the word discus, or in Greek it was diskos.
Perhaps we should open the question to all of you self-appointed linguists out there. What is the real distinction, if any, between the two forms?