Why different unit symbols - amu, u and dalton are used for representing atomic mass? And what does the term 'unified' mean in "unified mass unit"? Thank you.

I saw in my textbook (NCERT class 11 part2 chemistry) that oxidation is defined as gain of oxygen or more electronegative element by a substance. After it was redefined and said as oxidation is loss of electron(s) by any species. Again it is being redefined and stated as oxidatio...