Molecular mass is defined as the:

 

Concept: Molecular mass is a number equal to the sum of the atomic masses of the atoms in a molecule. 

The molecular mass gives the mass of a molecule relative to that of C¹² the atom, which is taken to have a mass of 12. 

Molecular mass is a dimensionless quantity, but it is given the unit Dalton or atomic mass unit as a means of indicating the mass is relative to 1$/$12  ^th the mass of a single atom of carbon-12.

Hence molecular mass is defined as the mass of one molecule of any substance compared with the mass of one atom of C – 12.