I do not know about definitions but here is choice of synonyms
121 Moby Thesaurus words for “orthodox”:
*ristian, Orthodox Jew, accepted, accordant, accustomed, acknowledged, admitted, anal, approved, authentic, authoritative, authorized, being done, bourgeois, button-down, canonical, canonist, comme il faut, common, compulsive, concordant, conformable, conformist, conservative, conventional, correct, corresponding, customary, de rigueur, decent, decorous, die-hard, doctrinal, dour, established, evangelical, faithful, firm, fogyish, formal, formalistic, fundamentalist, hard, harmonious, hidebound, impliable, in accord, in keeping, in line, in step, inexorable, inflexible, iron, ironbound, ironclad, ironhanded, kosher, literal, meet, muscle-bound, obdurate, obstinate, of the faith, official, old-line, ordinary, orthodox *ristian, orthodoxical, orthodoxist, pedantic, plastic, popular, precisianistic, prevailing, prevalent, procrustean, proper, purist, puristic, puritan, puritanic, reactionary, received, recognized, regular, relentless, right, rigid, rigorist, rigoristic, rigorous, rockbound, sanctioned, scriptural, seemly, sound, square, standard, stiff, straight, straightlaced, straitlaced, stubborn, stuffy, textual, textualist, textuary, the orthodox, tory, traditional, traditionalist, traditionalistic, true, true believer, true-blue, unbending, uncompromising, unrelenting, unyielding, uptight
Source: Moby Thesaurus II by Grady Ward, 1.0
And here is the “authoritative” definition by Wikipedia
Definitions of modern orthodox judaism on the Web: