A secretarius was a person, therefore, overseeing business confidentially, usually for a powerful individual (a king, pope, etc.).
When a communist party is in power, the general secretary is usually the country's de facto leader (though sometimes this leader also holds state-level positions to monopolize power, such as a presidency or premiership in order to constitute de jure leadership of the state), such as China, North Korea, Vietnam, Laos and Cuba.
[1] In England, the term secretarius was used "from the beginning of the thirteenth century in the varying meanings of a confidential clerk, an ambassador, or a member of the king's council".
[2] From the Renaissance to the late 19th century, men involved in the daily correspondence and the activities of the powerful assumed the title of secretary.
In a club or society, the secretary is also considered to be, in most cases, the third person in charge of the organization, after the president/chairman and vice president/vice chairman.