Short answers, pulled from the story.
Measurement is the quantification of an attribute of an object or event, used to compare it with other objects or events. It is the process of determining how large or small a physical quantity is compared to a basic reference quantity of the same kind, or the comparison of an unknown quantity with a known or standard one.
The seven SI base units are the second, metre, kilogram, ampere, kelvin, mole, and candela. Each is tied to a defining constant, such as the speed of light for the metre, the Planck constant for the kilogram, and the hyperfine splitting in caesium-133 for the second.
Charles Sanders Peirce, who lived from 1839 to 1914, made the first proposal to tie an SI base unit to an experimental standard independent of a physical artefact. He proposed defining the metre in terms of the wavelength of a spectral line, an idea that directly influenced the Michelson-Morley experiment.
The General Conference on Weights and Measures, known as the CGPM, was established in 1875 by the Metre Convention. It oversees the International System of Units, and through it the metre was redefined in 1983 in terms of the speed of light and the kilogram in 2019 in terms of the Planck constant.
Mass is the intrinsic property of all material objects to resist changes in their momentum, while weight is the downward force produced when a mass sits in a gravitational field. In free fall, with no net gravitational forces, objects lack weight but retain their mass.
In quantum mechanics, a measurement is an action that determines a particular property such as position, momentum, or energy of a quantum system. These measurements are always statistical samples from a probability distribution, and the most common interpretation holds that the wavefunction collapses to a single, definite value when a measurement is performed.