1. The quality of being fixed in place as by some firm attachment
   "The fastness of the bolt ensured the door wouldn't come loose";
   - fixedness, fixity, fixture, secureness
    
2. A strongly fortified defensive structure
   "The medieval fastness withstood many sieges";
   - stronghold
    
3. A rate (usually rapid) at which something happens
   "The fastness of the computer's processing impressed the engineers";
   - speed, swiftness

See also: fast, slow

Type of: defence [Brit, Cdn], defense [US], defensive structure, immovability, immovableness, pace, rate