Members of the military often use their own slang for a variety of things. The Army and Air Force share a lot of slang, while the sea services — Navy, Marine Corps and Coast Guard — use many of the same terms.
Ranks and Titles
In the Navy and Coast Guard, a senior chief petty officer is often called senior. In the Marine Corps, a gunnery sergeant is called gunny, a master sergeant is called top, a master gunnery sergeant can be called master guns, and those in the warrant officer ranks are gunners. A Marine Corps captain is often called skipper, but a Navy skipper refers to a captain of a ship. All Marines can be called leathernecks or devil dogs.
In the Army and Air Force, any enlisted member with the rank of sergeant and above can informally be called sarge, with an Army master sergeant and an Air Force senior master sergeant sometimes called top sergeant.
Enlisted recruits in the Navy are trained by recruit division commanders. Enlisted Marines are trained by drill instructors, while officers are trained by sergeant instructors. Service members in the Coast Guard are trained by company commanders, and those in the Army and Air Force are trained by drill sergeants.
Deployments
The Army and Marine Corps share some terms when deployed or in the field, aka downrange. Distances on maps are measured in clicks, which means kilometers. Infantrymen in both services are called grunts. When they're hiking through the field, soldiers are said to be on a ruck march while Marines are on a hump.
When setting sail on a cruise or float, the Navy and Coast Guard use maps called charts, and distances are measured in nautical miles.
Ships and Structures
The sea services use nautical terms to describe ships and structures — the Coast Guard calls its vessels cutters. The deck is the floor, walls are bulkheads, a bathroom is called the head, and beds are racks or bunks. When facing the bow or front of a vessel, the left side is called the port, and the right side is called the starboard.
In the Marine Corps, the dining facility is the chow hall or mess hall, and in the Navy, it's the galley.
In the Army and Air Force, the dining facility is often abbreviated to DFAC, and the bathroom is the latrine.
Other Slang
If someone gets into trouble, but it's not serious enough for them to be court martialed, the service member can receive nonjudicial punishment, aka Article 15 of the Uniform Code of Military Justice, or simply Article 15. Marines also call NJP office hours.
When Marines are given special liberty that's not part of their vacation time, known as leave, it usually involves an extended weekend, with three days off called a 72 and four days off a 96. The other service branches refer to it as a pass.
Being squared away means someone is acting professional and looking sharp in uniform in all services.
There are thousands of other slang terms used by the military, some of which are used by certain military occupational specialties, aka MOSs or jobs.