Hmac wrote:AR15 is a generic term for semi-automatic version of the M16 and it's subsequent variants, including the M4. It's an old term that Armalite used for what was to become the M16. When Colt bought the design, they trademarked AR15 for the semi-automatic version.
To nitpick just a little, the term AR15 was trademarked by Armalite, and predates both Colt acquiring the patent rights and the military adopting it and designating it as the M16. The original AR15s manufactured by Colt, both before and after they acquired the patent rights from Armalite were in fact select fire rifles. In fact there were a number of military owned select fire rifles that were roll marked AR15, US Govt Property instead of M16 after the military's adoption of the rifle.