a sweetheart chosen to receive a greeting on Saint Valentine's Day
"will you be my valentine?"
a card sent or given (as to a sweetheart) on Saint Valentine's Day
"She carefully wrapped the handwritten valentine in tissue paper before slipping it into his pocket for lunch."
An expression of affection, especially romantic affection, usually in the form of greeting card, gift, or message given the object of one's affection, especially on February 14th.
"She sent a card with her own handwritten note instead of buying a store-bought valentine for the holiday."
Alternative form of valentine
In plain English: A valentine is a card or gift given to someone you have a crush on, usually around Valentine's Day.
"She wrote a sweet note to her boyfriend for Valentine's Day."
Saint Valentine
"The history teacher explained that while many people celebrate a romantic holiday, our local church honors Saint Valentine as a martyr who stood up for love against injustice."
The word valentine comes from Saint Valentine's Day, which is celebrated as the day of romantic love. It entered English to refer specifically to a message or gift exchanged on that occasion.