Often shortened to "Flight", the flight director leads mission control
during manned and unmanned space
He is ultimately responsible for making critical mission decisions and is a key factor in the success of a mission, especially when things go wrong.
The position is so critical that (certainly for NASA manned missions, and possibly elsewhere as well), the flight director can not be over ruled or second guessed for the duration of a mission by anyone, except by being fired on the spot.
Famous American flight directors include Chris Kraft, Gene Kranz and John Hodge.