Submarine (noun)

1. A watercraft designed to operate underwater.
2. A naval vessel that is capable of submerging and operating underwater.
3. A type of boat or ship that is designed to be operated below the surface of the water.

Origin:

From sub- + marine, meaning "a watercraft designed to operate underwater.".

Examples:

1. The submarine was used for research missions in the ocean.
2. The submarine was equipped with torpedoes for military use.
3. The submarine was able to stay underwater for several days.
4. The submarine was part of a fleet of submarines.
5. The submarine was instrumental in winning the war.