The Collaborative International Dictionary of English v.0.48:

Infidel \In"fi*del\, a. [L. infidelis; pref. in- not + fidelis
   faithful, fr. fides faith: cf. F. infid[`e]le. See
   Fidelity.]
   Not holding the faith; -- applied by Christians to one who
   does not believe in the inspiration of the Scriptures, and
   the supernatural origin of Christianity; used by Mohammedans
   to refer to those who do not believe in Islam.
   [1913 Webster +PJC]

         The infidel writer is a great enemy to society. --V.
                                                  Knox.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Infidel \In"fi*del\, n.
   One who does not believe in the prevailing religious faith; a
   heathen; a freethinker; -- used especially by Christians and
   Mohammedans.
   [1913 Webster +PJC]

   Note: Infidel is used by English writers to translate the
         equivalent word used Mohammedans in speaking of
         Christians and other disbelievers in Mohammedanism.

   Syn: Infidel, Unbeliever, Freethinker, Deist,
        Atheist, Sceptic, Agnostic.

   Usage: An infidel, in common usage, is one who denies
          Christianity and the truth of the Scriptures. Some
          have endeavored to widen the sense of infidel so as to
          embrace atheism and every form of unbelief; but this
          use does not generally prevail. A freethinker is now
          only another name for an infidel. An unbeliever is not
          necessarily a disbeliever or infidel, because he may
          still be inquiring after evidence to satisfy his mind;
          the word, however, is more commonly used in the
          extreme sense. A deist believes in one God and a
          divine providence, but rejects revelation. An atheist
          denies the being of God. A sceptic is one whose faith
          in the credibility of evidence is weakened or
          destroyed, so that religion, to the same extent, has
          no practical hold on his mind. An agnostic remains in
          a state of suspended judgment, neither affirming nor
          denying the existence of a personal Deity.
          [1913 Webster]

WordNet (r) 3.0 (2006):

infidel
    n 1: a person who does not acknowledge your god [syn: heathen,
         pagan, gentile, infidel]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "infidel":
   agnostic, atheist, atheistic, disbeliever, disbelieving, ethnic,
   faithless, freethinker, gentile, giaour, goy, goyish, heathen,
   heretic, infidelic, kaffir, minimifidian, misbelieving,
   non-Christian, non-Jew, non-Jewish, non-Mohammedan, non-Mormon,
   non-Moslem, non-Muhammadan, non-Muslim, nonbeliever, nullifidian,
   pagan, profane, secularist, shegets, shiksa, unbeliever,
   unbelieving, unchristian, uncircumcised, zendician, zendik,
   zendikite



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

INFIDEL, persons, evidence. One who does not believe in the existence of a
God, who will reward or punish in this world or that which is to come.
Willes' R. 550. This term has been very indefinitely applied. Under the name
of infidel, Lord Coke comprises Jews and heathens; 2 Inst 506; 3 Inst. 165;
and Hawkins includes among infidels, such as do not believe either in the
Old or New Testament. Hawk. P. C. b 2, c. 46, s. 148.
     2. It is now settled that when the witness believes in a God who will
reward or punish him even in this world he is competent. See willes, R. 550.
His belief may be proved from his previous declarations and avowed opinions;
and when he has avowed himself to be an infidel, he may show a reform of his
conduct, and change of his opinion since the declarations proved when the
declarations have been made for a very considerable space of time, slight
proof will suffice to show he has changed his opinion. There is some
conflict in the cases on this subject, some of them are here referred to:
18 John. R. 98; 1 Harper, R. 62; 4 N. Hamp. R. 444; 4 Day's Cas. 51; 2
Cowen, R. 431, 433 n., 572; 7 Conn. R. 66; 2 Tenn. R. 96; 4 Law Report, 268;
Alis. Pr. Cr. Law, 438; 5 Mason, 16; 15 mass. 184; 1 Wright, 345; So. Car.
Law Journ. 202. Vide Atheist; Future state.



The Devil's Dictionary (1881-1906):

INFIDEL, n.  In New York, one who does not believe in the Christian
religion; in Constantinople, one who does.  (See GIAOUR.)  A kind of
scoundrel imperfectly reverent of, and niggardly contributory to,
divines, ecclesiastics, popes, parsons, canons, monks, mollahs,
voodoos, presbyters, hierophants, prelates, obeah-men, abbes, nuns,
missionaries, exhorters, deacons, friars, hadjis, high-priests,
muezzins, brahmins, medicine-men, confessors, eminences, elders,
primates, prebendaries, pilgrims, prophets, imaums, beneficiaries,
clerks, vicars-choral, archbishops, bishops, abbots, priors,
preachers, padres, abbotesses, caloyers, palmers, curates, patriarchs,
bonezs, santons, beadsmen, canonesses, residentiaries, diocesans,
deans, subdeans, rural deans, abdals, charm-sellers, archdeacons,
hierarchs, class-leaders, incumbents, capitulars, sheiks, talapoins,
postulants, scribes, gooroos, precentors, beadles, fakeers, sextons,
reverences, revivalists, cenobites, perpetual curates, chaplains,
mudjoes, readers, novices, vicars, pastors, rabbis, ulemas, lamas,
sacristans, vergers, dervises, lectors, church wardens, cardinals,
prioresses, suffragans, acolytes, rectors, cures, sophis, mutifs and
pumpums.