The Collaborative International Dictionary of English v.0.48:

Ultra \Ul"tra\, a. [See Ultra-.]
   Going beyond others, or beyond due limit; extreme; fanatical;
   uncompromising; as, an ultra reformer; ultra measures.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Ultra \Ul"tra\, n.
   One who advocates extreme measures; an ultraist; an
   extremist; a radical. --Brougham.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Ultra- \Ul"tra-\, a.
   A prefix from the Latin ultra beyond (see Ulterior), having
   in composition the signification beyond, on the other side,
   chiefly when joined with words expressing relations of place;
   as, ultramarine, ultramontane, ultramundane, ultratropical,
   etc. In other relations it has the sense of excessively,
   exceedingly, beyond what is common, natural, right, or
   proper; as, ultraconservative; ultrademocratic,
   ultradespotic, ultraliberal, ultraradical, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ultra
    adj 1: (used of opinions and actions) far beyond the norm;
           "extremist political views"; "radical opinions on
           education"; "an ultra conservative" [syn: extremist,
           radical, ultra]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "ultra":
   Bolshevik, Jacobin, Wobbly, anarchist, anarcho-syndicalist,
   at the height, at the limit, extreme, extreme left-winger,
   extremist, fanatic, far out, furthest, greatest, kinky,
   left-wing extremist, lunatic fringe, mild radical, most, nihilist,
   outre, parlor Bolshevik, parlor pink, pink, pinko, rabid, radical,
   red, revolutional, revolutionary, revolutionist, sans-culotte,
   subversive, syndicalist, too much, ultra-ultra, ultraist, utmost,
   uttermost, way out, yippie