Qapla' is thlIngan Hol (Klingon) for "Success". As a phrase, it works along the lines of "Have a nice day.", "Good luck.", or "Y'all come back now, hear?". (TLE novel: The Art of the Impossible)
It is used in the name of this ship:
Miscellaneous[]
The more formal farewell to the tera'ngan translation of, "Die honorably!" Qapla' is roughly equivalent to the Romulan jolan tru and Vulcan Live long and prosper. (The Klingon Way: A Warrior's Guide , Star Trek: The Klingon Book of Virtues)