Assistant Secretary of Defense is a title used for many executive positions in the United States Department of Defense. The United States Department of Defense, abbreviated as DoD or DOD and sometimes called the Defense Department, is a civilian Cabinet organization of the United States government. ...

They include (among several others):

• Assistant Secretary of Defense for Public Affairs
• Assistant Secretary of Defense for Health Affairs
• Assistant Secretary of Defense for International Security Policy
• Assistant Secretary of Defense for Reserve Affairs
• Assistant Secretary of Defense for Special Operations and Low-Intensity Conflict
• Assistant Secretary of Defense for Legislative Affairs

These positions are not to be confused with the office of Deputy Secretary of Defense. Unlike the assistant secretaries, there is only one deputy secretary, and his office is similar in nature to that of the full Secretary of Defense The United States Deputy Secretary of Defense is the second-highest ranking official in the United States Department of Defense. ... The United States Secretary of Defense is the head of the United States Department of Defense, concerned with the armed services and The Secretary is appointed by the President with the approval of the Senate, and is a member of the Cabinet. ...