Shaken – (pronounced Sha as in SHU in the word Shut and ken as in the name Ken).  Got it?

Shaken is simply car registration.  
From the time a car (for private use) is first registered it has three years shaken/registration and from there out the registration is to be renewed evry two years for the life of the car (if to be used on Japan roads).  

Japanese Car Registration Paper – “Shaken Sho” – Example

The ShakenSho (or Shaken Paper) is the car’s registration paper. This paper shows the car’s registration number (number plate number), chassis number, year and month of first registration in Japan, name of the owner of the car and the owner’s address and other data.

* You must keep the Shaken-Sho in your vehicle at all times  along with the Jibaiseki – Japanese compulsory insurance document.