drill instructor

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A noncommissioned officer who instructs recruits in military drill and discipline.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun US The generic term for the principal instructor of fresh military recruits undergoing basic training in the various branches of the United States military, routinely abbreviated to the initialism DI; always filled by enlisted personnel (a sergeant or petty officer).

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a noncommissioned officer who instructs recruits in military marching and discipline