Kilogram is the fundamental unit of mass and it is equal to 2.205 pound(lb). First it was defined as mass of 1 litre (cm^3) water which was then modified in 1889, as a cylinder made of 90% Platinum and 10% Iridium named as an International Prototype of the Kilogram (IPK). In 2019, a kilogram was redefined in terms of three fundamental physical constants:
1: Speed of Light 'c'
2: Specific Atomic Transition Frequency '∆Vcs'
3: Planks Constant 'h'
It is now defined as taking the fix numerical value of planck constant h to be 6.626*10^-34 when expressed in J.s which is equal to kg.m^2.s.-1, where metre and second are defined in terms of 'c' and '∆Vcs' res.
This definition makes kilogram consistent with the older definition : the mass remains within 30 PPM of the mass of 1 litre of water.