CINC (sink) -
n. military acronym for Commander IN Chief.
The person at the top of your chain of command, the boss. Usually refers to the President, but can refer to other, lesser commanders. The US Navy, for example, has CINCPACFLT (sink-pack-fleet), who is the CINC of the Pacific Fleet, who takes orders from CINCPAC, the commander of all military operations in the Pacific Ocean and surrounding countries.
Recently, Dick Cheney, the Secretary of Defense, mandated that the phrase "CINC" only refer to the President of the United States, and specifically is not to be used to describe theater or regional commanders. This policy is somewhat unpopular because it undoes a whole bunch of Navy acronyms. It will likely be reversed next time we have a new SECDEF.