The Collaborative International Dictionary of English v.0.48:

Headquarters \Head"quar`ters\ (-kw[add]r`t[~e]rz), n. pl. [but
   sometimes used as a n. sing.]
   1. The quarters or place of residence of any chief officer,
      as the general in command of an army, or the head of a
      police force; the place from which orders or instructions
      are issued; hence, the center of authority or order.
      [1913 Webster]

            The brain, which is the headquarters, or office, of
            intelligence.                         --Collier.
      [1913 Webster]

   2. The main office from which an organization such as a
      commercial enterprise is managed; -- usually where the
      chief executive officer works.
      [PJC]

WordNet (r) 3.0 (2006):

headquarters
    n 1: (usually plural) the office that serves as the
         administrative center of an enterprise; "many companies
         have their headquarters in New York" [syn: headquarters,
         central office, main office, home office, home
         base]
    2: the military installation from which a commander performs the
       functions of command; "the general's headquarters were a
       couple of large tents" [syn: headquarters, HQ, military
       headquarters]
    3: (plural) a military unit consisting of a commander and the
       headquarters staff