Etymology
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impartial (adj.)
"not partial, not favoring one over another," 1590s, from assimilated form of in- (1) "not, opposite of" + partial. First recorded use is in "Richard II." Related: Impartially.
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impartiality (n.)
"fairness, freedom from bias," 1610s; see impartial + -ity.
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unbiased (adj.)
c. 1600, literal, in reference to throws at bowls, from un- (1) "not" + biased. Figurative sense of "impartial, unprejudiced" is recorded from 1640s.
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even-handed (adj.)
also evenhanded, "impartial, equitable, rightly balanced," c. 1600, from even (adj.) + -handed. Related: even-handedly; even-handedness.
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dispassionate (adj.)

1590s, of persons, "free from passions, calm, disposed;" 1640s, "not dictated by passion, impartial;" from dis- "the opposite of" + passionate. Related: Dispassionately.

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disinterested (adj.)

1610s, "unconcerned" (the sense that now would go with uninterested), from dis- "opposite of" + interested. The sense of "impartial" originally was in disinteressed (c. 1600), from Old French desinteresse, and subsequently passed to uninterested. The modern sense of disinterested, "impartial, free from self-interest or personal bias, acting from unselfish motives," is attested by 1650s.

By late 18c. the words had sorted themselves out, and as things now stand, disinterested means "impartial," uninterested means "caring nothing for the matter in question," and disinteressed has fallen by the wayside. Related: Disinterestedly; disinterestedness.

Disinterested and uninterested are sometimes confounded in speech, though rarely in writing. A disinterested person takes part in or concerns himself about the affairs of others without regard to self interest, or to any personal benefit to be gained by his action; an uninterested one takes no interest in or is indifferent to the matter under consideration .... [Century Dictionary]
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indifferent (adj.)
late 14c., "unbiased, impartial, not preferring one to the other" (of persons), "alike, equal" (of things), from Old French indifferent "impartial" or directly from Latin indifferentem (nominative indifferens) "not differing, not particular, of no consequence, neither good nor evil," from in- "not, opposite of" (see in- (1)) + differens, present participle of differre "set apart" (see differ). Extended sense of "apathetic, no more inclined to one thing than to another" first recorded early 15c.; that of "neither good nor bad" is from 1530s, on notion of "neither more nor less advantageous," but since 17c. it has tended toward "rather bad."
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equiparation (n.)
mid-15c., "impartial treatment;" 1610s, "equal ranking;" from Latin aequiparationem (nominative aequiparatio) "an equalizing, comparison," from past participle stem of aequiparare "put on equality, compare," from aequipar "equal, alike," from aequus "equal, even" (see equal (adj.)) + par (see par (n.)). Related: Equiparate.
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iso- 
before vowels often is-, word-forming element meaning "equal, similar, identical; isometric," from Greek isos "equal to, the same as; equally divided; fair, impartial (of persons); even, level (of ground)," as in isometor "like one's mother." In English used properly only with words of Greek origin; the Latin equivalent is equi- (see equi-).
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equal (adj.)
late 14c., "identical in amount, extent, or portion;" early 15c., "even or smooth of surface," from Latin aequalis "uniform, identical, equal," from aequus "level, even, flat; as tall as, on a level with; friendly, kind, just, fair, equitable, impartial; proportionate; calm, tranquil," which is of unknown origin. Parallel formation egal (from Old French egal) was in use late 14c.-17c. Equal rights is from 1752; by 1854 in American English in reference to men and women. Equal opportunity (adj.) in terms of hiring, etc. is recorded by 1925.
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