From article below:
"The final version of the airship will be about 450 feet long. However, the prototype will be only a third of that size. The craft is known to military planners as ISIS, or Integrated Sensor Is the Structure, referring to the radar system built into the structure of the craft.
The ISIS has a hull made of a lightweight, thick skin. Zeppelins - like the Hindenburg - have a rigid external structure. Blimps are not rigid, and are given their shape from the pressure of the helium gas. According to military, the ISIS is closer to a blimp than a zeppelin, but officials most frequently call it an airship. "Airship," like "dirigible," is a broader term."