[1] : The Collaborative International Dictionary of English v.0.48

invalid \inval"id\, a. pref. in- not + valid: cf. f. invalide,
   l. invalidus infirm, weak. cf. invalid infirm.
   1913 webster
   1. of no force, weight, or cogency; not valid; weak.
      1913 webster

   2. law having no force, effect, or efficacy; void; null;
      as, an invalid contract or agreement.
      1913 webster
see also:
invalid 

[2] : The Collaborative International Dictionary of English v.0.48

invalid \in"valid\ ?; 277, n. f. invalide, n.  a., l.
   invalidus, a. see invalid null.
   a person who is weak and infirm; one who is disabled for
   active service; especially, one in chronic ill health who is
   unable to care for himself.
   1913 webster
see also:
invalid 

[3] : The Collaborative International Dictionary of English v.0.48

invalid \in"valid\, a. see invalid, n.
   not well; feeble; infirm; sickly; as, he had an invalid
   daughter.
   1913 webster
see also:
invalid 

[4] : The Collaborative International Dictionary of English v.0.48

invalid \in"valid\, v. t.
   1. to make or render invalid or infirm. "invalided, bent, and
      almost blind." --dickens.
      1913 webster

   2. to classify or enroll as an invalid.
      1913 webster

            peace coming, he was invalided on half pay.
                                                  --carlyle.
      1913 webster

[5] : WordNet (r) 2.0

invalid
     adj 1: having no cogency or legal force; "invalid reasoning"; "an
            invalid driver's license" ant: valid
     2: no longer valid; "the license is invalid"
     n : someone who is incapacitated by a chronic illness or injury
         syn: shut-in
     v 1: force to retire, remove from active duty, as of firemen
     2: injure permanently; "he was disabled in a car accident"
        syn: disable, incapacitate, handicap
see also:
valid shut-in disable incapacitate handicap 

[6] : Moby Thesaurus II by Grady Ward, 1.0

194 moby thesaurus words for "invalid":
   diogenes, hieronymian, hieronymite, timon of athens, absonant,
   afflict, ailing, anchoress, anchorite, annulled, apoplectic,
   arthritic, ascetic, bad, barren, bedridden invalid, black,
   blank cartridge, bootless, cachectic, case, castrato,
   cloistered monk, closet cynic, consumptive, contradictory,
   contrary to reason, counterproductive, criminal, cripple,
   debilitate, debilitated, derange, desert fathers, desert saints,
   devitalize, disable, disabled, disorder, drained, dud, dyspeptic,
   effete, empty, enervate, enervated, enfeeble, epileptic, eremite,
   erroneous, etiolated, eunuch, evil, exhausted, failing, fallacious,
   false, fatuitous, fatuous, faulty, feckless, feeble, flawed, frail,
   fruitless, futile, gelding, healthless, hermit, hermitess,
   homebody, hospitalize, ill, illogical, impaired, imperfect,
   impotent, improper, in poor health, inaccurate, inadequate, inane,
   inauspicious, inauthentic, incapacitate, incompetent, inconclusive,
   incongruous, inconsequent, inconsequential, inconsistent,
   incorrect, incurable, indispose, ineffective, ineffectual,
   inefficacious, inexpedient, inferior, infirm, inoperative,
   inpatient, irrational, isolationist, languishing, lay up, loner,
   loose, mad, malevolent, marabout, moribund, nonrational,
   nonscientific, not following, nugacious, nugatory, null and void,
   of no force, outcast, outpatient, pale, paralogical, pariah,
   patient, peaked, peaky, peccant, pillar saint, pillarist,
   reasonless, recluse, reduce, reduced, reduced in health, repealed,
   repudiated, revoked, rheumatic, run-down, seclusionist,
   self-annulling, self-contradictory, self-refuting, senseless,
   shut-in, sick, sick person, sicken, sickly, sinful, sinister,
   solitaire, solitary, solitudinarian, sophistic, spastic, spurious,
   stay-at-home, sterile, stylite, sufferer, terminal case, the sick,
   unauthentic, unavailing, unconnected, unfavorable, unhealthy,
   unkind, unphilosophical, unpleasant, unreasonable, unscientific,
   unskillful, unsound, untenable, untoward, untrue, useless, vain,
   valetudinarian, valetudinary, vicious, victim, void, weaken,
   weakened, weakling, weakly, wicked, with low resistance,
   without reason, wrong

[7] : Bouvier's Law Dictionary, Revised 6th Ed (1856)

invalid. in a physical sense, it is that which is wanting force; in a 
figurative sense, it signifies that which has no effect. 

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Invalid, definition of term: Invalid
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