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.
   [1913 Webster]

         Seasonable mementos may be useful.       --Bacon.
   [1913 Webster]

WordNet (r) 3.0 (2006):

memento
    n 1: a reminder of past events [syn: memento, souvenir]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "memento":
   arch, barrow, boundary stone, brass, bust, cairn, cenotaph, column,
   commemoration, cromlech, cross, cup, cyclolith, dolmen, favor,
   footstone, grave, gravestone, headstone, hoarstone, inscription,
   keepsake, marker, mausoleum, megalith, memento mori, mementos,
   memorabilia, memorial, memorial arch, memorial column,
   memorial statue, memorial stone, memorials, memories, menhir,
   monolith, monument, mound, necrology, obelisk, obituary, pillar,
   plaque, prize, pyramid, relic, reliquary, remembrance,
   remembrancer, reminder, ribbon, rostral column, shadow, shaft,
   shrine, souvenir, stela, stone, stupa, tablet, testimonial, token,
   token of remembrance, tomb, tombstone, tope, trace, trophy