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1.
A Master Sergeant is a Marine that has mastered the specific skillset for a job, and is hand chosen to remain in this field. Master Sergeants conduct operations and are ready for deployment. Master Sergeants are subject experts and are looked upon for guidance and advice on actual operations. Master Sergeants are sexy beasts.

Of equal rank but opposite in job function is the First Sergeant. First Sergeants are closest to the person in charge but are actually responsible for only administrative tasks for their organization. First Sergeants make sure all the trash is picked up and rooms are clean. Their function is similar to that of a nanny or personal assistant.

A Master Sergeant outranks Managers, Asst Managers, General Leaders, Honchos, and El Jeffe.

A Director outranks a Master Sergeant in all matters.
The Colonel says:

"Get the Master Sergeants, we are preparing for deployment."

"Get the First Sergeant, there is trash in the common areas."
by TheOtherLeeroy November 13, 2011
 
2.
Rank in the army that is above Sergeant First Class, but below First Sergeant. Master Segeants usually serve as staff directors rather than field officers.
Mike was the Master Sergeant in charge of supply
by baaaaah May 12, 2005