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"

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "ex officio":
   absolute, ascendant, authoritarian, authoritative, authorized,
   autocratic, clothed with authority, commanding, competent,
   consequential, considerable, controlling, dominant,
   duly constituted, eminent, empowered, ex cathedra, governing,
   great, hegemonic, hegemonistic, imperative, important, influential,
   leading, mighty, momentous, monocratic, official, potent, powerful,
   preeminent, prestigious, prominent, puissant, ranking, ruling,
   senior, substantial, superior, supreme, totalitarian, weighty



Bouvier's Law Dictionary, Revised 6th Ed (1856):

EX OFFICIO. By virtue of his office. 2. Many powers are granted and
exercised by public officers which are not expressly delegated. A judge, for
example, may, ex officio, be a conservator of the peace, and a justice of
the peace.