Etymology
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relation (n.)

c. 1300, relacioun, "relationship, connection, correspondence;" late 14c. as "act of telling or relating in words," from Anglo-French relacioun, Old French relacion "report, connection" (14c.) and directly from Latin relationem (nominative relatio) "a bringing back, restoring; a report, proposition," from relatus (see relate).

The meaning "person related by blood or marriage" is attested from c. 1500. The phrase no relation "not in the same family," used in differentiating persons with the same surname, is attested by 1930.

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relational (adj.)

1660s, "of or pertaining to human relations," from relation + -al (1). From 1840 as "indicating or specifying some relation" in general. Related: Relationality.

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relationship (n.)

1640s, "sense or state of being related" by kindred, affinity, or other alliance, from relation + -ship. Specifically of romantic or sexual intimacy by 1944.

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correlation (n.)

1560s, "mutual relation, interdependence, interconnection," from French corrélation, from cor- "together" (see com-) + relation (see relation). Meaning "action of bringing into orderly connection" is by 1879.

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proportion (n.)

late 14c., proporcioun, "due relation of one part to another," also "size, extent; comparative relation of one thing to another in size, degree, number, etc.," from Old French proporcion "measure, proportion" (13c.) and directly from Latin proportionem (nominative proportio) "comparative relation, analogy," from phrase pro portione "according to the relation" (of parts to each other), from pro "for" (see pro-) + ablative of *partio "division," related to pars "a part, piece, a share, a division" (from PIE root *pere- (2) "to grant, allot"). Also from late 14c. as "relation of body parts," hence "form, shape." Phrase out of proportion attested by 1670s.

My fortunes [are] as ill proportioned as your legs. [John Marston, "Antonio and Mellida," 1602]
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interrelate (v.)
also inter-relate, 1831 (implied in interrelated), transitive, "bring into reciprocal relation," from inter- "between" + relate (v.). Intransitive sense "come into reciprocal relation" is attested from 1912. Related: Interrelating.
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bereavement (n.)
"grievous loss," especially the death of a friend or close relation, 1731, from bereave + -ment.
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misstatement (n.)

"a wrong statement, an erroneous account or relation," 1783, from misstate + -ment or else from mis- + statement.

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complementarity (n.)

"a complementary relation or situation," 1908, a term in physics, from complementary + -ity.

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