Linchpin
From Wikipedia, the free encyclopedia
A linchpin (occasionally lynchpin) is a metal part used in mechanical engineering to prevent a wheel or other rotating part from sliding off the axle it is riding on.
In colloquial speech, it is used to refer to the object at the center of a certain situation. An example of this is the main character in a sitcom, who acts as the glue that keeps all the other characters and situations together.