What term is used for the closing of a letter?
Solution 1:
The symmetrical term is valediction, but I don't suppose it's very much used.
Solution 2:
I've only ever heard that line referred to as the closing.
If we're to go by one of the many virtually identical letter diagrams available online, that usage is confirmed, although some call it the complimentary closing.
Barrie's suggestion of valediction is technically correct, but so far as I can tell it's not really in use.