B.D.U.

From the Metal Gear Wiki

B.D.U. (short for Battle Dress Uniform) is a uniform commonly used in the military. One B.D.U. outfit was given to Raiden by Mr. X during the Big Shell Incident so that he could infiltrate the Big Shell's Shell One Core. The disguise didn't work on it's own, however, he needed an AKS-74u to complete the disguise.

[edit] In-Game

While both disguise items are equipped (the B.D.U. and the AK-74), never do anything to attract attention (such as crawling or rolling) and never un-equip either of the items. Try not to bump into any guards because this will cause Raiden to accidentally un-equip the B.D.U.

[edit] Trivia

• Pliskin and The Colonel both point out that the B.D.U.'s measurements are closer to Rosemary's size than they are Raiden's. This is an early hint that Mr. X is in fact Olga Gurlukovich.