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Entry 2 senses Webster, 1913

Valentine

/văl'-ən-tīnˌ/ · Val·en·tine · IPA /ˈvaləntaɪn/
01 n. A sweetheart chosen on St. Valentine's Day.
  1. 1.
    A sweetheart chosen on St. Valentine's Day.
  2. 2.
    A letter containing professions of love, or a missive of a sentimental, comic, or burlesque character, sent on St. Valentine's Day.
Phrases & compounds
St. Valentine's Day — a day sacred to St. Valentine; the 14th of February. It was a very old notion, alluded to by Shakespeare, that on this day birds begin to mate. Hence, perhaps, arose the custom of sending love tokens at that time.