What is Ministry?

Legal Definition
A ministry is a governmental organisation, headed by a minister, that is usually meant to manage a specific sector of public administration. Ministries have a bureaucratic structure.

Different states have different number of ministries. Brockhaus and Efron Encyclopedic Dictionary notes that all states have Ministry of Interior, Ministry of Foreign Affairs, Ministry of Defense (that can be divided into ministries for land forces and navy), Ministry of Justice and Ministry of Finance. Ministry of Education is also common.

Ministries are usually an immediate subdivision of the Cabinet (i.e. the executive branch of the government), and subordinate to its chief executive who is either called prime minister, chief minister, president, minister-president or (federal) chancellor.

In the 20th century, many Western countries (including monarchies such as Belgium and the United Kingdom) have trended away from the usage of the designation "ministry", preferring to replace it partially or entirely with words such as department, office, state secretariat, public service, or even agency and bureau. In some countries, these terms may be used with specific meanings, for example an office may be a subdivision of a department.
-- Wikipedia
Legal Definition
Office; service. Those members of the government who are In the cabinet.
-- Black's Law Dictionary