In physics, the solid state is one of the three phases of matter (solid, liquid, and gas). See also: solid state physics.
In electronics, solid state circuits are those that do not contain vacuum tubes. The term was introduced to describe the transition from valve-based amplifiers to transistorised amplifiers. Solid-state components last much longer than their thermionic counterparts because of their greater resistance to shock, vibration and mechanical wear.
As the use of vacuum tubes in consumer electronics has all but disappeared, the term "solid state" has been increasingly used as a synonym for "no moving parts." For example, digital audio players that store all their songs in flash memory are often described as solid state to differentiate them from iPod-like devices that use small hard disks for storage. Like the older usage, this usage connotes increased durability and shock resistance.
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