What is "A" in AGeV? AGeV means GeV per nucleon. But why A letter is used for such a short cut? Why not NGeV, for example? 
 A: The $A$ is a variable; it's actually $A\text{ GeV}$, where $A$ is the mass number.
For example, RHIC has run Au-Au collisions at a beam energy of $100A\text{ GeV}$ (among other values). You would interpret this as $100A$ being the number of GeV. With gold nuclei, $A = 197$, and so that is $(100\times 197)\text{ GeV} = 19700\text{ GeV}$ total energy per nucleus.
A: This is an artifact of a long established coding for nuclei


*

*$Z$ -- the "atomic number" is the number of protons in the nucleus(or occasionally the "proton number") 

*$A$ -- the "mass number" the number of nucleons in the nucleus


There is also the not often used


*

*$N$ -- the "neutron number"


The per-nucleon energy of course applies to both the protons and the neutrons, so you want $A$.

And yeah, the use of these letters doesn't make much sense, but it has been that way for a long, long time.
A: N.GeV would be confusing as the symbol N is commonly used to denote the number of neutrons.
From the Wikipedia article on Atomic Number:

The atomic number, Z, should not be confused with the mass number, A,
  which is the number of nucleons, the total number of protons and
  neutrons in the nucleus of an atom. The number of neutrons, N, is
  known as the neutron number of the atom; thus, A = Z + N.

