1. 
   Rapidity
   In physics, rapidity is a parameter of the unit hyperbola used to relate frames of reference in special relativity. The term "rapidity" in English is usually a synonym of speed, but in special relativity rapidity is a continuous group parameter of the Lorentz group. Essentially, rapidity is a hyper...
   Found op http://en.wikipedia.org/wiki/Rapidity
   
   
2. 
   Rapidity
   Ra·pid'i·ty (rȧ*pĭd'ĭ*tȳ) noun [ Latin rapiditas : confer French rapidité .] The quality or state of being rapid; swiftness; celerity; velocity; as, the rapidity of a current; rapidity of speech;...
   Found op http://www.encyclo.co.uk/webster/R/11
   
   
3. 
   Rapidity
   • (n.) The quality or state of being rapid; swiftness; celerity; velocity; as, the rapidity of a current; rapidity of speech; rapidity of growth or improvement.
   Found op http://thinkexist.com/dictionary/meaning/rapidity/
   
   
4. 
   rapidity
   rapidity A rapid state or quality; speediness; celerity.
   Found op http://www.wordinfo.info/words/index/info/view_unit/1830/
   
   
5. 
   rapidity
   1) Alacrity 2) Celerity 3) Dispatch 4) Fastness 5) Fleetness 6) Pace 7) Quickness 8) Rate 9) Speed 10) Speediness 11) Swiftness 12) Velocity
   Found op http://www.mijnwoordenboek.nl/EN/crossword-dictionary/rapidity/1
   
   

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