In physics and chemistry, "AMU" is an abbreviation for atomic mass unit. Sometimes, it is further abbreviated as "u" for unified atomic mass unit. One AMU is the mass of one-twelfth of an atom of the element carbon, isotope 12, and is equal to 1.661 x 10^-27 kilograms.
Carbon-12 is used as the base measure of an AMU to ensure that the new calculated values of atomic masses are close to the values that appear in older texts. The previous fundamental measurement had been in terms of the 16th isotope of oxygen, which has a mass of exactly 16 AMU.