The Collaborative International Dictionary of English v.0.48:

Perplexity \Per*plex"i*ty\, n.; pl. Perplexities. [L.
   perplexitas: cf. F. perplexit['e].]
   The quality or state of being perplexed or puzzled;
   complication; intricacy; entanglement; distraction of mind
   through doubt or difficulty; embarrassment; bewilderment;
   doubt.
   [1913 Webster]

         By their own perplexities involved,
         They ravel more.                         --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

perplexity
    n 1: trouble or confusion resulting from complexity

Moby Thesaurus II by Grady Ward, 1.0:

138 Moby Thesaurus words for "perplexity":
   Chinese puzzle, Gordian knot, amorphousness, baffle, bafflement,
   baffling problem, befuddlement, bewilderment, bind, bother,
   botheration, brain twister, catch-22, chaos, cloud, complexity,
   complexness, complication, confoundment, confusion, convolution,
   crabbedness, crossword puzzle, crux, dark, darkness, daze,
   difficulty, dilemma, discombobulation, discomfiture, discomposure,
   disconcert, disconcertedness, disconcertion, disconcertment,
   disorder, disorganization, disorientation, distress, disturbance,
   doubt, embarrassment, enigma, enigmatic question, entanglement,
   fix, floorer, flummox, flurry, fluster, flutter, fog, fogginess,
   frenzy, fuddle, fuddlement, fuzziness, haze, impenetrability,
   indeterminateness, indistinctness, intricacy, intricateness,
   involution, involvement, jam, jigsaw puzzle, jumble, knot,
   knotty point, maze, mess, mind-boggler, mist, mistiness, muddle,
   muddlement, mumbo jumbo, murk, murkiness, mystery, mystification,
   node, nodus, nonplus, nut to crack, obfuscation, obscurantism,
   obscuration, obscurity, opacity, oxymoron, paradox,
   perplexed question, perturbation, pickle, plight, pons asinorum,
   poser, pother, predicament, problem, pucker, puzzle, puzzlement,
   puzzler, quandary, question, question mark, ramification,
   reconditeness, riddle, ruffle, scrape, shapelessness, shuffle,
   sixty-four dollar question, stew, sticker, stumper, subtlety,
   sweat, swivet, tanglement, teaser, technicality, tizzy,
   tough proposition, unassuredness, unclarity, unclearness,
   unplainness, unsettlement, upset, vagueness, vexed question, why



The Free On-line Dictionary of Computing (30 December 2018):

perplexity

   The geometric mean of the number of words which may follow
   any given word for a certain lexicon and grammar.