Head, chief, director, president, principal, chief executive, chair, manager; supervisor, foreman, overseer, controller, authority figure; employer, owner, proprietor; number one, kingpin, bossman, boss lady, top dog, bigwig, big cheese, head honcho, and big kahuna.

Supervisors, managers, directors, If all none of the above apply, you can always refer to your boss as "EMPLOYER"

If you are addressing him directly, it would be Sir, Mr. Smith, Ms. Smith, Chief, Boss (the latter two are somewhat colloquial). Sometimes you can call your boss by his first name, but if you do so, still act like you know he is your boss.

Manager or supervisor (like Lauren said, if they are )

You could refer to your boss as your superior or manager (if they are).