Valentine \Val"en*tine\, n.

1. A sweetheart chosen on St. Valentine's Day.

2. A letter containing professions of love, or a missive of a sentimental, comic, or burlesque character, sent on St. Valentine's Day.

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.

Source: Webster's Revised Unabridged Dictionary (1913)