Etymology
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observation (n.)

late 14c., observacioun, "the performance of a religious rite," from Old French observation (c. 1200) and directly from Latin observationem (nominative observatio) "a watching over, observance, investigation," noun of action from past-participle stem of observare "watch over, note, heed, look to, attend to, guard, regard, comply with," from ob "in front of, before" (see ob-) + servare "to watch, keep safe," from PIE root *ser- (1) "to protect." Sense of "act or fact of paying attention" is from 1550s. Meaning "a remark in reference to something observed" is recorded from 1590s.

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observational (adj.)

1834, "of or pertaining to (scientific) observation," from observation + -al (1).

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observance (n.)

mid-13c., observaunce, "act performed in accordance with prescribed usage," especially a religious or ceremonial one; late 14c., "care, concern, act of paying attention (to something)," from Old French observance, osservance "observance, discipline," and directly from Latin observantia "act of keeping customs, attention, respect, regard, reverence," from observantem (nominative observans), present participle of observare "watch over, note, heed, look to, attend to, guard, regard, comply with," from ob "in front of, before" (see ob-) + servare "to watch, keep safe," from PIE root *ser- (1) "to protect." Observance is the attending to and carrying out of a duty or rule. Observation is watching, noticing.

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remark (n.)

1650s, "fact of being worthy of comment," also "an act of observation" (a sense now obsolete), from remark (v.). The meaning "a verbal or written notice or comment" is from 1670s; the sense of "observation, notice" also is from 1670s.

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extraspection (n.)
"outward observation," 1887, from extra- + ending from introspection.
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memoirs (n.)

"personal record of events, narrative of the facts or events of the life of a person or a phase of history written from personal knowledge or observation upon points about which the writer is specially informed," 1650s, plural of memoir.

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speculation (n.)
late 14c., "intelligent contemplation, consideration; act of looking," from Old French speculacion "close observation, rapt attention," and directly from Late Latin speculationem (nominative speculatio) "contemplation, observation," noun of action from Latin speculatus, past participle of speculari "observe," from specere "to look at, view" (from PIE root *spek- "to observe").

Meaning "pursuit of the truth by means of thinking" is from mid-15c. Disparaging sense of "mere conjecture" is recorded from 1570s. Meaning "buying and selling in search of profit from rise and fall of market value" is recorded from 1774; short form spec is attested from 1794.
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experience (n.)
Origin and meaning of experience

late 14c., "observation as the source of knowledge; actual observation; an event which has affected one," from Old French esperience "experiment, proof, experience" (13c.), from Latin experientia "a trial, proof, experiment; knowledge gained by repeated trials," from experientem (nominative experiens) "experienced, enterprising, active, industrious," present participle of experiri "to try, test," from ex "out of" (see ex-) + peritus "experienced, tested," from PIE *per-yo-, suffixed form of root *per- (3) "to try, risk." Meaning "state of having done something and gotten handy at it" is from late 15c.

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scopophilia (n.)
"voyeurism," 1924 (in a translation of Freud), from Greek -skopia "observation" (from PIE root *spek- "to observe") + -philia. In early use often scoptophilia through a mistake by Freud's translators. Modern form by 1937. Related: Scopophiliac.
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picket (v.)

1745, "to enclose or fortify with pointed stakes," from picket (n.). Meaning "to place or post as a guard of observation" is by 1775. The sense in labor strikes, protests, etc., is attested from 1867. Related: Picketed; picketing.

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