noun
board chairman; managing director; chief director
The top executive of an organization's board, such as a school foundation, hospital, or non-profit. In some contexts, may be translated as 'president' or 'chairperson'.
彼は学校法人の理事長に就任した。
He assumed the position of board chairman of the educational foundation.
理事長が会議を招集した。
The managing director convened the meeting.
社長 is the president of a company, whereas 理事長 is the head of a board of directors, often in non-profit or educational institutions.
Compound of 理事 (director, board member) and 長 (head, chief).