general secretary; secretary-general; chief secretary
Used for the top administrative officer of a political party, labor union, or similar organization. The exact English title depends on the organization's conventions.
He was elected general secretary of the party.
The union's secretary-general issued a statement.
事務局長 is often used for the head of a secretariat or administrative office, similar in rank but more bureaucratic than 書記長, which is common in political parties and unions.
Compound of 書記 (secretary, clerk) and 長 (head, chief).