The Collaborative International Dictionary of English v.0.48:

Memento \Me*men"to\, n.; pl. Mementos. [L., remember, be
   mindful, imper. of meminisse to remember. See Mention.]
   A hint, suggestion, token, or memorial, to awaken memory;
   that which reminds or recalls to memory; a souvenir.
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         Seasonable mementos may be useful.       --Bacon.
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