What is Atomic Mass Unit?

Atomic mass unit is also referred as a.m.u .

It is defined as the mass which is exactly 1/12^th of the mass of a ₆C¹² atom.

Thus,
1 a.m.u = 1/12^th of mass of ₆C¹²

= (1/12)*12g mol^-1 /N_A

= 1g mol ^-1 / (6.022*10²³mol^-1)

= 1.66*10^-23g