Refers to the body of directors as a group, not the meeting itself.
取締役会は新しい社長を選任した。
The board of directors appointed a new president.
board meeting; meeting of the board of directors
Refers to the meeting itself. Context usually makes the distinction clear.
来週の取締役会でその件を話し合う予定です。
We plan to discuss that matter at next week's board meeting.
It was decided that an emergency board meeting would be held next week.
The appointment of the new president was decided at the board meeting.
This investment plan must obtain the board of directors' approval.
Outside directors also have a duty to attend board meetings.