noun (n)

• the quality of being fixed in place as by some firm attachment(noun.attribute)

  Synonym:
  fastness, fixedness, fixture, secureness
  Antonym:
  looseness
  

  source: wordnet30
• the quality of being incapable of mutation(noun.attribute)

  Example:
  Darwin challenged the fixity of species.
  

  Synonym:
  immutability, immutableness
  Antonym:
  mutability, mutableness
  

  source: wordnet30
• Fixedness; as, fixity of tenure; also, that which is fixed.(noun)

  source: webster1913