Source : Webster's Revised Unabridged Dictionary (1913)

Ex officio \Ex` of*fi"ci*o\; pl. {Ex officiis}. [L.]
   From office; by virtue, or as a consequence, of an office;
   officially.

Source : WordNet®

ex officio
     adj : by virtue of an office or position; "the head of the
           department serves as an ex officio member of the board"
     adv : by virtue of position; "the president sat on the committee
           ex officio" [syn: {by right of office}]