WordNet (r) 3.0 (2006):

solid state
    n 1: the state in which a substance has no tendency to flow
         under moderate stress; resists forces (such as compression)
         that tend to deform it; and retains a definite size and
         shape [syn: solid, solidness, solid state]

The Free On-line Dictionary of Computing (30 December 2018):

solid state

    (Often hyphenated when used as an adjective) A
   term describing any device that uses semiconductor
   components rather than electron tubes or relays.
   Semiconductors are "solid" in that they contain no gas
   ("vacuum" tubes contain a small amount as the vacuum is not
   perfect) or moving parts (like relays), but probably more
   important is the connotation of reliability and durability
   that made possible things like portable radios.

   (2007-09-21)