The Scientific definition of mass

What is the definition of mass in Science ?

The Scientific definition of mass   

The kilogram is defined in terms of a platinum–iridium standard mass kept near Paris. For measurements on an atomic scale, the atomic mass unit, defined in terms of the atom carbon-12, is usually used.

The international 1 kg standard of
mass, a platinum–iridium cylinder 3.9 cm in
height and in diameter.

The SI standard of mass is a cylinder of platinum and iridium  that is kept at the International Bureau of Weights and Measures near Paris and assigned, by

international agreement, a mass of 1 kilogram. Accurate copies have been sent to standardizing laboratories in other countries, and the masses of other bodies can be determined by balancing them against a copy. 

The U.S. copy of the standard kilogram is housed in a vault at NIST. It is removed, no more than once a year, for the purpose of checking duplicate copies that are used elsewhere. Since 1889, it has been taken to France twice for re-comparison with the primary standard.