The term "submarine," as an adjective, simply means under the sea. But as a noun, a submarine invokes the mental image of a boat that can wreak havoc during wartime through its stealth and power. Although they are large craft that are crewed by over 150 submariners, a submarine is always referred to as a "boat." That's because during their development, the name of the craft was shortened from the adjective "submarine boat" to create the noun "submarine."