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1. The quality of being fixed in place as by some firm attachment
   - fastness, fixedness, fixture, secureness
    
2. The quality of being incapable of mutation
   "Darwin challenged the fixity of species";
   - immutability, immutableness

Derived forms: fixities

Type of: changelessness, immovability, immovableness, unchangeability, unchangeableness, unchangingness

Antonym: looseness, mutability

Encyclopedia: Fixity