Originally Posted by sukalp we can say quantity is just a amount or number.
mass is a quantity of matter present inside our body.
quantity are atoms only.
as matter is made up of atoms. 
To be honest I get very disappointed when what I've already posted is ignored with no reason. If you claim its wrong then please explain why!
I explained that in
modern physics, which includes relativity, nongravitational fields are also considered to be matter. This definition was given by Einstein when he published general relativity in which he wrote in his 1916 GR review paper
We make a distinction between "gravitational field" and "matter" in this way, that we denote everything but the gravitational field as "matter." Our use of the word includes not only matter in the ordinary sense, but the electromagnetic field as well.

The use of the term "mass" to denote the EM field dates back to early special relativity. It was published in Einstein's article
The Principle of Conservation of the Center of Gravity and the Inertia of Energy, Albert Einstein,
Annalen der Physik, 20 (1906): 626633.
Specifically in this article Einstein wrote
If we assign the electromagnetic field too a mass density ...

This use of mass to refer to fields and even photons is quite prominent in a great many modern relativity textbooks.
If can prove that wrong, please do so. Otherwise please stop ignoring it and implying or claiming otherwise. Thank you.