Etymology
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memorial (adj.)

late 14c., "memorable, excellent," also "remembered, committed to memory," from Old French memorial "mindful of, remembering" (Modern French mémorial), and directly from Latin memorialis "of or belonging to memory," from memoria "memory" (from PIE root *(s)mer- (1) "to remember"). From mid-15c. as "preservative of memory, serving for commemoration."

A Middle English word for "having to do with memory" was memorative (late 14c.), from Old French memoratif, from Latin memorativus. Though useful, it apparently has not survived.

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memorial (n.)

late 14c., "fame, renown, reputation," also "a commemorative gesture, monument, or rite;" in general, "something by which the memory of a person, thing, or event is preserved," from Old French memorial "record, report," and directly from Late Latin memoriale "a memorial," noun use of neuter of Latin memorialis (adj.) "of or belonging to memory," from memoria "memory" (from PIE root *(s)mer- (1) "to remember").

Meaning "memorial act, commemorative gesture, monument, etc.," is from late 14c., as is the sense of "a written representation of facts made to a legislative or other body as the grounds for a petition." Related: Memorialist.

The old Man said, “I see around me here
Things which you cannot see: we die, my Friend,
Nor we alone, but that which each man loved
And prized in his peculiar nook of earth
Dies with him or is changed, and very soon
Even of the good is no memorial left.
[Wordsworth, "The Ruined Cottage"]
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memorialize (v.)

"to present a memorial to, petition by memorial," 1708; "to commemorate, preserve the memory of," 1765; from memorial + -ize. Related: Memorialized; memorializing.

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Memorial Day 

"day on which a memorial is made," by 1819, of any anniversary date, especially a religious anniversary; see memorial (adj.). As a specific end-of-May holiday commemorating U.S. war dead, it began informally in the late 1860s and originally commemorated the Northern soldiers killed in the Civil War. It was officially so called by 1869 among veterans' organizations, but Decoration Day also was used. The Grand Army of the Republic, the main veterans' organization in the North, officially designated it Memorial Day by resolution in 1882:

That the Commander-in-Chief be requested to issue a General Order calling the attention of the officers and members of the Grand Army of the Republic, and of the people at large, to the fact that the proper designation of May 30th is Memorial Day and to request that it may be always so called. [Grand Army Blue Book, Philadelphia, 1884]

The South, however, had its own Confederate Memorial Day, and there was some grumbling about the apparent appropriation of the name.

The word "Memorial" was adopted by the Maryland Confederates shortly after the war, and has been generally used throughout the South. It is distinctively Confederate in its origin and use, and I would suggest to all Confederate societies to adhere to it. The Federals' annual day of observance is known as "Decoration Day," having been made so by an act of Congress, and the 30th day of May named as the date. In Maryland there is annually a Decoration Day and a Memorial Day. The two words are expressive not only of the nature of the observance, but also of the people who participate therein. [Confederate Veteran, November 1893]
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immemorial (adj.)
c. 1600, from French immémorial "old beyond memory" (16c.), from Medieval Latin immemorialis, from assimilated form of in- "not, opposite of" (see in- (1)) + Latin memorialis of or belonging to memory" (see memorial (n.)). Something immemorial is ancient beyond memory; something immemorable is not worth remembering. Latin immemor meant "unmindful, forgetful, heedless."
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*(s)mer- (1)

Proto-Indo-European root meaning "to remember." 

It forms all or part of: commemorate; commemoration; mourn; memo; memoir; memorable; memorandum; memorial; memorious; memorize; memory; remember.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit smarati "remembers;" Avestan mimara "mindful;" Greek merimna "care, thought," mermeros "causing anxiety, mischievous, baneful;" Latin memoria "memory, remembrance, faculty of remembering," memor "mindful, remembering;" Serbo-Croatian mariti "to care for;" Welsh marth "sadness, anxiety;" Old Norse Mimir, name of the giant who guards the Well of Wisdom; Old English gemimor "known," murnan "to mourn, remember sorrowfully;" Dutch mijmeren "to ponder."

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festschrift (n.)
"volume of writings by various scholars presented as a tribute or memorial to a veteran scholar," 1898, from German Festschrift, literally "festival writing" (see -fest + script (n.)).
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monument (n.)

late 13c., "a sepulchre," from Old French monument "grave, tomb, monument," and directly from Latin monumentum "a monument, memorial structure, statue; votive offering; tomb; memorial record," literally "something that reminds," a derivative of monere "to remind, bring to (one's) recollection, tell (of)," from PIE *moneie- "to make think of, remind," suffixed (causative) form of root *men- (1) "to think." Meaning "any enduring evidence or example" is from 1520s; sense of "structure or edifice to commemorate a notable person, action, period, or event" is attested from c. 1600.

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obit (n.)

late 14c., "death," a sense now obsolete, from Old French obit or directly from Medieval Latin obitus "death" (a figurative use, literally "a going down, a going to a place"), noun use of past participle of Latin obire "to die," literally "to go toward" (see obituary).

From c. 1400 as "anniversary of a person's death; memorial service held on the anniversary of a person's death." In modern usage (since 1874) it is usually a clipped form of obituary, though it had the same meaning of "published death notice" 15c.-17c. The scholarly abbreviation ob. with date is from Latin obiit "(he) died," third person singular of obire.

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representation (n.)

c. 1400, representacioun, "image, likeness symbolic memorial," from Old French representacion (14c.) and directly from Latin repraesentationem (nominative repraesentatio), "a bringing before one, a showing or exhibiting," noun of action from past-participle stem of repraesentare "show, exhibit, display" (see represent (v.)).

The sense of "act of presenting to the mind or imagination" is attested by 1640s. The meaning "statement made in regard to some matter" is from 1670s. Legislative sense of "fact of representing or being represented" is by 1769, thus "share or participation in legislation, etc., by means of regularly chosen or appointed delegates; the system by which communities and societies have a voice in their own affairs and the making of their laws."

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