The Collaborative International Dictionary of English v.0.48:

Ex officio \Ex` of*fi"ci*o\; pl. Ex officiis. [L.]
   From office; by virtue, or as a consequence, of an office;
   officially.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ex officio
    adv 1: by virtue of position; "the president sat on the
           committee ex officio" [syn: ex officio, by right of
           office]
    adj 1: by virtue of an office or position; "the head of the
           department serves as an ex officio member of the board"