A military assistant, in the British Armed Forces and many of those derived from them, is an officer appointed to the personal office of a general officer. They are similar to aides-de-camp but generally have a more overtly administrative role.
Equivalent
In the Canadian Forces, the equivalent position is that of Executive Assistant.
In the United States Department of Defense, a military assistant is a military officer serving as aide to very senior civilian (typically a presidentai appointee in OSD or in the service secretariats), while a military officer in an equivalent position serving a general/flag officer is an executive assistant.
References
Military assistant Wikipedia(Text) CC BY-SA