Weight definition unit and instrument?
In the SI the unit of mass is kilogram: the mass of of a cylinder (height=diameter=
39,17 mm) made from 90 % platinum and 10 % iridium.
The instrument to measure mass is a balance.
Weight itself is not a unit. It what we would measure in terms of pounds, ounces. These are units of weight. +++ Note that this uses the common definition of 'weight'. Strictly speaking, the pound, ounce, gramme, etc are all units of mass, not weight. Weight is the force produced by the mass subject purely to the acceleration due to gravity (9.8m/s^2 or 32ft/s^2 on Earth).