Hardened plasma

Hardened plasma is a strange thing. Calling it "plasma" is somewhat inacurrate though, as in actuality it is not.
Hardened plasma is essentially a magnetically charged gas, ejected, confined into a magnetic field, and then rapidly compressed to a solid. This allows it to be molded into whatever shapes necessary, while simultaneously being extremely tough. The drawbacks, then, are the energy needed for both the magnetic field and keeping the material at the state it is in. The gas itself is usually drawn from whatever atmosphere is present.
The material itself is very ice-like, except it isn't cold. It's also very white and opaque.

Utility

Weapons, and emergency structures. Due to the strength of the material, as well as the ease of shaping it, it has many, many applications.
Weapons are a reasonably common one, as they are easy to transport. Hardened plasma melee weapons are less common than regular ones, and they have their pros and cons. On the plus side, they're smaller, and easier to transport. Hardened plasma is tougher than Carbon-Interlaced Aluminium Alloy, while being lighter. Downsides include the fact that you can only really keep a blade on for a small amount of time, usually only when striking. It's very energy-intensive on a portable platform. Cutting power isn't relevant here, as you can still put a Layered Chain Blade on hardened plasma. Another downside is that the shapes can't be very complex, as the magnetic field can only be shaped to a limit.
It's also used for shields, for vehicles. These aren't your typical scifi shields though, as they restrict visibility and can't be shot through. Thus, they're deployed right before impact, which is also the reason lasers will always go through, if only for a bit before shields are deployed. For ground vehicles, the shield is as a cover, following the vehicles shape close to it. In space, in zero-g, they are more classic. A big bubble, or at least parts of it. In space shields can also be layered, and are, with a space in between. The bigger a ship, the more layers its shields have. Stringer's flagship, the The Orator of Ten Thousand Glorious Victories has six. There's also infantry-scale portable walls of hardened plasma.
It can also be used for emergency structures, like a support pillar. Since it's so durable and strong, it can handle a lot of weight, and connected to a city's power grid, it can be kept on for a longer while, often hours, enough to, say, evacuate people and important equipment.

Access & Availability

Expensive, but common. Basic assembly devices can't really make them, you need some more advanced stuff to do so.

Complexity

Even despite being quite complex, the machinery isn't very big.

Discovery

Discovered in the 2390s, it only really got popular a few decades later.