One atomic mass unit is equal to weight of

One atomic mass unit $=\frac{1}{12}\text{th of\hspace{0.17em}C}-12$ isotope of carbon. 

Atomic mass unit (a.m.u): In terms of carbon $-12$, it is defined as “ the quantity of mass equal to $\frac{1}{12}\text{th}$ of the mass of carbon $-12$ atom”.