1570s, "existing on both sides, exclusive or interchangeable" (of duties, etc.), with -al (1) + stem of Latin reciprocus "returning the same way, alternating," from pre-Latin *reco-proco-, from *recus (from re- "back;" see re-, + -cus, adjective formation) + *procus (from pro- "forward," see pro-, + -cus). Related: Reciprocally.

From 1590s as "given, felt, or shown in return;" c. 1600 as "corresponding or answering to each other, mutually equivalent." The sense of "moving backward and forward, having an alternating back and forth motion" (c. 1600) is obsolete. The noun meaning "that which is reciprocal" (to another) is from 1560s. In scientific and mechanical uses, reciprocating, reciprocative (1804), and reciprocatory (1826) have been tried.

1520s, "a reflexive mode of expression;" 1560s, "act of making a return (especially if mutual), mutual giving and returning, interchange of acts," from Latin reciprocationem (nominative reciprocatio) "retrogression, alternation, ebb," noun of action from past-participle stem of reciprocare "move back, turn back," also "come and go, move back and forth;" from reciprocus "returning the same way; alternating" (see reciprocal).

1540s as a chemical term in reference to alternating vaporization and condensation, from Latin circulatus, past participle of circulare "to form a circle," from circulus "small ring" (see circle (n.)).

Intransitive sense of "to pass about freely, pass from place to place or person to person" is from 1660s; of newspapers from 1885. Of blood, "to flow in a continuous circuit," from 1650s; of persons, "to mingle in a social gathering," from 1863. Related: Circulated; circulating.

"normal rhythmic relaxation of the heart" (alternating with the systole), 1570s, from medical Latin diastole, from Greek diastole "drawing asunder, dilation," from diastellein, from dia "through; thoroughly, entirely" (see dia-) + stellein "to set in order, arrange, array, equip, make ready," from PIE *stel-yo-, suffixed form of root *stel- "to put, stand, put in order," with derivatives referring to a standing object or place. Related: Diastolic.

1610s, "to give and return mutually," a back-formation from reciprocation, or else from Latin reciprocatus, past participle of reciprocare "rise and fall, move back and forth; reverse the motion of," from reciprocus "returning the same way, alternating" (see reciprocal). Sense of "cause to move back and forth" is from 1650s; intransitive sense of "move backward and forward" is from 1670s. Meaning "to give or do in response, act in return or response" is from 1820. Related: Reciprocated; reciprocating.

1580s, "alternating trade wind of the Indian Ocean," from Dutch monssoen, from Portuguese monçao, from Arabic mawsim "time of year, appropriate season" (for a voyage, pilgrimage, etc.), from wasama "he marked." The Arabic word, picked up by Portuguese sailors in the Indian Ocean, was used for anything that comes round every year (such as a festival), and was extended to the season of the year when the monsoon blows from the southwest (April through October) and the winds were right for voyages to the East Indies. In India, the summer monsoon is much stronger than the winter and was popularly spoken of emphatically as "the monsoon." It also brings heavy rain, hence the meaning "heavy episode of rainfall during the rainy season" (1747). Related: Monsoonal.