Mass and weight are two different terms:
Mass: Mass of a body is the quantity of matter contained in it. It is never zero for a material body
and it is not affected by the presence of other bodies. The unit of mass in the MKS and SI system is kg.
Weight: The weight of a body is the force with which it is attracted towards the centre of the earth.It
becomes zero at the centre of the earth and it is affected by the presence of other bodies. The unit of
weight in MKS is kg wt. and in SI is newton (N).
Relation between mass and weight
Weight = Mass of the body ( m )x acceleration due to gravity (g)
w = mg (value of g is taken as 9·81 m/sec2 in MKS and S.I. system)