It is defined as the ratio of substance to the  given material

It is represented by a greek alphabet...
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The Brainliest Answer!
it is defined as the ratio of the mass of certain volume of the substance to the mass of an equal volume of water at 4 degree celcius . It is a number and has no unit . 
    since R.D. is the ratio of two similar quantities 
     R.D. of mercury = density of Hg / density of water at 4 degree celcius 

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