Mossad (literally translated as The Institute) is Israel's civilian foreign intelligence and covert action organization, as opposed to Aman which is the Israeli military intelligence service and the Shin Bet, which is responsible for internal security. The full name is The Institute for Intelligence and Special Operations (Hebrew: המוסד למודיעין ולתפקידים מיוחדים).
In the fictional NCIS series, Ziva David, long before she became an NCIS Special Agent worked for Mossad and was assigned to Leroy Jethro Gibbs's team as a Liaison Officer for both NCIS and Mossad. She worked with the team for three or four years before rejoining Mossad at the end of Season 6 after her boyfriend, Michael Rivkin was shot dead by Anthony DiNozzo.
But in the Season 7 premiere, Ziva quit working for Mossad for good and succeeded in becoming an NCIS Special Agent.