Interlocking directorate are corporate boards that share one or more directors.
SecuritiesCE Explains Interlocking Directorate
High level corporate executives will often be asked to join the board of directors of other corporations. These executives bring a high level of expertise in their field and can be a great asset for the corporation. It is not unusual for these executives to be asked to serve on the boards of several companies. When two corporations share one or more members of their boards the corporations are said to have an interlocking directorate.