Difference between atomic mass, mass number, atomic weight, and relative atomic mass
Please correct me if I am wrong but in my understanding, the following are the definitions for the aforementioned terms:

Mass number: The total number of nucleons present in the nucleus of an atom(Dimensionless). 

Atomic mass: The combined mass of all the nucleons present in the atom of any given element(Unit: amu/u).

Relative isotopic mass: Ratio of atomic mass to one amu(Dimensionless)

Average atomic mass: Weighted average of all the isotopes for the given element(amu/u).

Relative atomic mass/atomic weight: Ratio of atomic weight to one amu(Dimensionless). 

Here is the Wikipedia page, explaining that "atomic weight" is simply the older term used for "relative atomic mass":

Relative atomic mass