What is lnu in air ticket?

I usually put LNU (Last Name Unknown) as the last name field whenever it is mandatory; either in flight reservation or visa. Answer 1 of 8: I had booked the ticket in Turkish Airlines and i write the LNU in surname field because in my passport only first name and surname is blank. LNU stands for last name Unknown. Is it mandatory to include a middle name in a flight ticket if the middle name is.