Weight
In modern scientific usage, weight and mass are fundamentally different quantities: mass is an intrinsic property of matter, whereas weight is a "force" that results from the action of gravity on matter: it measures how strongly the force of gravity pulls on that matter. However, in most practical everyday situations the word "weight" is used when, strictly, "mass" is meant. For example, most people would say that an object "weighs one kilogram", even though the kilogram is a unit of mass.
Kilogram
Gravity and the nature of the Kibble balance, which oscillates test masses up and down against the local gravitational acceleration "g", are exploited so that mechanical power is compared against electrical power, which is the square of voltage divided by electrical resistance. However, "g" varies significantly—by nearly 1%—depending on where on the Earth's surface the measurement is made (see "Earth's gravity"). There are also slight seasonal variations in "g" at a location due to changes in underground water tables, and larger semimonthly and diurnal changes due to tidal distortions in the Earth's shape caused by the Moon and the Sun. Although "g" would not be a term in the "definition" of the kilogram, it would be crucial in the process of measurement of the kilogram when relating energy to power. Accordingly, "g" must be measured with at least as much precision and accuracy as are the other terms, so measurements of "g" must also be traceable to fundamental constants of nature. For the most precise work in mass metrology, "g" is measured using dropping-mass absolute gravimeters that contain an iodine-stabilized helium–neon laser interferometer. The fringe-signal, frequency-sweep output from the interferometer is measured with a rubidium atomic clock. Since this type of dropping-mass gravimeter derives its accuracy and stability from the constancy of the speed of light as well as the innate properties of helium, neon, and rubidium atoms, the 'gravity' term in the delineation of an all-electronic kilogram is also measured in terms of invariants of nature—and with very high precision. For instance, in the basement of the NIST's Gaithersburg facility in 2009, when measuring the gravity acting upon Pt10Ir test masses (which are denser, smaller, and have a slightly lower center of gravity inside the Kibble balance than stainless steel masses), the measured value was typically within 8 ppb of .