In physics, electronics, etc. “solid state” means either something that has no moving parts (solid state drive) or that has no liquid or gas (solid state laser).
Why add the word “state”? Doesn’t the word “solid” on its own suffice?
A block of metal is solid, a machine is solid state. Does that help?
For electronics, at least, “Solid State” was used to differentiate between the newer semi-conductor and older vacuum tube technologies. Primarily just because physically, the new devices (transistors, diodes, etc) were grown from crystals, as opposed to vacuum tubes which worked with clouds of electrons.
I always view it as a reference to states of matter. In solid state device, everything takes place in a solid state, and not another state. You’re probably familiar with liquids, gases, and solids, and maybe even plasmas, and the (new since I was edumacated) Bose-Einstein state.
One could argue that electro-mechanical devices are solid state, but I suppose one could also argue that movement occurs through air or vacuum.
Physicists tend to talk about condensed matter.
Solid state is, as Khendrask notes, a colloquial term that was intended to differentiate electronics - transistors and other semiconductor devices (diodes are not specific to solid state - you can have a thermionic diode) that work by controlling the flow of electrons (or the lack of electrons) in a solid - from electronic devices that relied upon controlling the flow of electrons in a vacuum. After that “solid state” became a synonym for things that contained transistors, or worked with transistors (and their many variants). So a solid state drive isn’t really talking about the lack of moving parts - it is referring to the fact that the storage medium is built out of transistors rather than magnetic material. However the idea that it does refer to a lack of moving parts is taking hold to some extent, and I would not be surprised to see holographic storage systems being referred to as “solid state” even though they neither use transistor based storage, nor are free of moving parts (the spatial modulator looks a lot like a DLP chip - which is anything but free of moving parts.)
The term ‘solid’ could also be construed to mean ‘composed of a single uniform material’ or ‘containing no voids’ - neither of which is (or was) particularly true of solid state electronic devices.
solid state diodes and transistors were replacement technology for vacuum tubes which contained lots of empty hollow space, that space could be referred to as a gaseous state (even though you actually wanted gases not to be there).
No, it’s not. We use “solid-state” all the time.
The term solid state is also used for aluminum electrolytic capacitors, which usually have a liquid electrolyte but some have a solid polymer based electrolyte (one could also say that ceramic, film and (most) tantalum capacitors are solid-state but usually it refers to aluminum electrolytics, the distinction being a much higher reliability, especially at higher temperatures).
Sort of proves my point, the OP was asking whether “solid state” referred to the device being a solid, without moving parts. The point is that it doesn’t. Solid state refers to its operation being performed by semiconductors.