Etymology
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memento (n.)

late 14c., "Psalm cxxxi in the Canon of the Mass" (which begins with the Latin word Memento and in which the dead are commemorated), from Latin memento "remember," second person singular imperative of meminisse "to remember, recollect, think of, bear in mind," a reduplicated form, related to mens "mind," from PIE root *men- (1) "to think." Meaning "a hint or suggestion to awaken memory, a reminder, an object serving as a warning" is from 1580s; sense of "keepsake" is recorded by 1768.

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momento (n.)
a misspelling, or perhaps variant, of memento.
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memento mori (n.)

"a reminder of death," 1590s, a decorative object, usually an ornament for the person, containing emblems of death or reminders of the fleetingness of life, common in 16c., a Latin phrase, literally "remember to die," that is, "remember that you must die." From second person singular imperative of meminisse "to remember, recollect, think of, bear in mind" (a reduplicated form, related to mens "mind," from PIE root *men- (1) "to think") + mori "to die" (from PIE root *mer- "to rub away, harm" (also "to die" and forming words referring to death and to beings subject to death).

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*men- (1)

Proto-Indo-European root meaning "to think," with derivatives referring to qualities and states of mind or thought.

It forms all or part of: admonish; Ahura Mazda; ament; amentia; amnesia; amnesty; anamnesis; anamnestic; automatic; automaton; balletomane; comment; compos mentis; dement; demonstrate; Eumenides; idiomatic; maenad; -mancy; mandarin; mania; maniac; manic; mantic; mantis; mantra; memento; mens rea; mental; mention; mentor; mind; Minerva; minnesinger; mnemonic; Mnemosyne; money; monition; monitor; monster; monument; mosaic; Muse; museum; music; muster; premonition; reminiscence; reminiscent; summon.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit manas- "mind, spirit," matih "thought," munih "sage, seer;" Avestan manah- "mind, spirit;" Greek memona "I yearn," mania "madness," mantis "one who divines, prophet, seer;" Latin mens "mind, understanding, reason," memini "I remember," mentio "remembrance;" Lithuanian mintis "thought, idea," Old Church Slavonic mineti "to believe, think," Russian pamjat "memory;" Gothic gamunds, Old English gemynd "memory, remembrance; conscious mind, intellect."

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commemorate (v.)

1590s, "call to remembrance," from Latin commemoratus, past participle of commemorare "bring to remembrance," from com-, here perhaps an intensive prefix (see com-), + memorare "to remind," from memor "mindful of" (from PIE root *(s)mer- (1) "to remember").

Meaning "perpetuate the memory of" (by solemn act, etc.) is from 1630s. Of things, "to serve as a memento of, to perpetuate the memory of," 1766. Related: Commemorated; commemorates; commemorating.

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souvenir (n.)
1775, "a remembrance or memory," from French souvenir (12c.), from Old French noun use of souvenir (v.) "to remember, come to mind," from Latin subvenire "come to mind," from sub "up from below" (see sub-) + venire "to come," from a suffixed form of PIE root *gwa- "to go, come." Meaning "token of remembrance, memento" is first recorded 1782.
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remembrance (n.)

c. 1300, remembraunce, "a memory, recollection," from Old French remembrance (11c.), from remembrer (see remember). From late 14c. as "consideration, reflection; present consciousness of a past event; store of personal experiences available to recollection, capacity to recall the past." Also late 14c. as "memento, keepsake, souvenir," and "a commemoration, remembering, ritual of commemoration." Meaning "faculty of memory, capability of remembering" is from early 15c.

British Remembrance Day, the Sunday nearest Nov. 11 (originally in memory of the dead of World War I) is attested from 1921. A remembrancer (early 15c.) was a royal official of the Exchequer tasked with recording and collecting debts due to the Crown; hence also, figuratively "Death" (late 15c.).

To us old lads some thoughts come home
Who roamed a world young lads no more shall roam.
[Melville, from "To Ned"]
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relic (n.)

c. 1200, relik, "a body part or other object held in reverence or affection due to its connection with a holy person," from Old French relique, relike (11c., plural reliques), from Late Latin reliquiæ (plural) "the remains of a martyr," in classical Latin "remains, remnants," noun use of the fem. plural of reliquus "remaining, that which remains."

This is related to relinquere (perfective reliqui) "to leave behind, forsake, abandon, give up," from re- "back" (see re-) + linquere "to leave" (from PIE *linkw-, nasalized form of root *leikw- "to leave").  Old English used reliquias, directly from Latin.

The general sense of "remains, remnants, that which is left after the loss or ruin of the rest" is attested from early 14c. The meaning "something kept as a souvenir, a memento" is from c. 1600. By 1590s the word had developed its weakened sense of "anything made interesting by its association with the distant past." By 1580s as "surviving trace of some practice, idea, etc.;" hence relic of barbarism (by 1809) "survival of a (bad) old custom or condition."

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