The Collaborative International Dictionary of English v.0.48:

Critical \Crit"ic*al\ (kr[i^]t"[i^]*kal), a. [See Critic, n.,
   Crisis.]
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   1. Qualified to criticise, or pass judgment upon, literary or
      artistic productions.
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            It is submitted to the judgment of more critical
            ears to direct and determine what is graceful and
            what is not.                          --Holder.
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   2. Pertaining to criticism or the critic's art; of the nature
      of a criticism; accurate; as, critical knowledge; a
      critical dissertation.
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   3. Inclined to make nice distinctions, or to exercise careful
      judgment and selection; exact; nicely judicious.
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            Virgil was so critical in the rites of religion,
            that he would never have brought in such prayers as
            these, if they had not been agreeable to the Roman
            customs.                              --Bp.
                                                  Stillingfleet.
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   4. Inclined to criticise or find fault; fastidious; captious;
      censorious; exacting.
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            O gentle lady, do not put me to 't,
            For I am nothing, if not critical.    --Shak.
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   5. Characterized by thoroughness and a reference to
      principles, as becomes a critic; as, a critical analysis
      of a subject.
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   6. [See Crisis.] Pertaining to, or indicating, a crisis,
      turning point, or specially important juncture; important
      as regards consequences; hence, of doubtful issue;
      attended with risk; dangerous; as, the critical stage of a
      fever; a critical situation.
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            Our circumstances are indeed critical. --Burke.
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            The small moment, the exact point, the critical
            minute, on which every good work so much depends.
                                                  --South.
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   Critical angle (Optics), that angle of incidence of a
      luminous ray at which it is wholly reflected, and no
      portion of it transmitted. The sine of this angle is the
      reciprocal of the refractive index of the medium.

   Critical philosophy, the metaphysical system of Kant; -- so
      called from his most important work, the "Critique of Pure
      Reason."
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WordNet (r) 3.0 (2006):

critical angle
    n 1: the smallest angle of incidence for which light is totally
         reflected