Source : Webster's Revised Unabridged Dictionary (1913)

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.

         By their own perplexities involved, They ravel more.
                                                  --Milton.

Source : WordNet®

perplexity
     n : trouble or confusion resulting from complexity

Source : Free On-Line Dictionary of Computing

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