Source : Webster's Revised Unabridged Dictionary (1913)

Weakness \Weak"ness\, n.
   1. The quality or state of being weak; want of strength or
      firmness; lack of vigor; want of resolution or of moral
      strength; feebleness.

   2. That which is a mark of lack of strength or resolution; a
      fault; a defect.

            Many take pleasure in spreading abroad the weakness
            of an exalted character.              --Spectator.

   Syn: Feebleness; debility; languor; imbecility; infirmness;
        infirmity; decrepitude; frailty; faintness.

Source : WordNet®

weakness
     n 1: a flaw or weak point; "he was quick to point out his wife's
          failings" [syn: {failing}]
     2: powerlessness revealed by an inability to act; "in spite of
        their weakness the group remains highly active" [syn: {helplessness},
         {impuissance}]
     3: the property of lacking physical or mental strength;
        liability to failure under pressure or stress or strain;
        "his weakness increased as he became older"; "the weakness
        of the span was overlooked until it collapsed" [ant: {strength}]
     4: the condition of being financially weak; "the weakness of
        the dollar against the yen" [ant: {strength}]
     5: a penchant for something even though it might not be good
        for you; "he has a weakness for chocolate"