I think that "persistant" is the key feature.

The only reason we don't have nuclear reactors in aircraft is in case they crash in the wrong place.

The reason we don't have reactors in airplanes is there is no value in flying for years when the mean time between failures of the equipment onboard is measured in hours.
There is no value in paying for all the pain of building an airplane that can carry months of provisions when it is never more than some hours away from a place where it can regenerate and maybe even swap crews.  A ship needs to think of being days or even weeks transit from port.  A spaceship traveling for years may not really be a ship anymore.  It may be more correct to call it an ark.

 

Treadgar

Something I haven't seen explored in scifi is the impact that the path a particular culture takes to space, and to a space force in particular could dramatically impact what they call their troops, and different cultures might well use different names.

 

One culture might have fleets commanded by admirals and another might have squadrons commanded by generals. Some space forces might consider their ground troops to be Army, others Marines. Some might consider them comparable to naval CBs, others to the USAF ground units like pararescue and pathfinders--the folks who land on a planet to set up infrastructure to support the space craft.

 

The limited endurance on spacecraft of the supplies needed by their crew might dictate that ships in a campaigning space force live off of the land, finding habitable planets to obtain supplies from. As such, a ship capable of deploying a space elevator could be a key component of a scifi fleet that is frequently overlooked.