Definition of line letter:
(n) :
A letter written to express the author's love for the intended reader.
(n) :
(figurative) A work that shows great passion or enthusiasm for something.
(n) :
(US, slang) A notice from the U.S. Customs and Border Protection informing about the seizure of a personal shipment as contraband.
(n) :
A thin snack of crisp baked dough eaten at Chinese New Year.