The three fundamental units of measurement for length, mass, and time are the meter, kilogram, and second, respectively. These are known as the International System of Units (SI) base units and are used as the basis for all other units of measurement.
Standards of measurement are determined and maintained by national and international organizations such as the National Institute of Standards and Technology (NIST) in the United States and the International Bureau of Weights and Measures (BIPM) globally. These organizations establish and maintain physical standards that represent the defined units of measurement.
The purpose of having standardized units of measurement is to provide a consistent and universal system for measuring quantities in order to facilitate accurate and reliable communication and comparison of data. It also allows for efficient scientific research and development, as well as international trade and commerce.
Standards of measurement have evolved over time through the refinement and improvement of measurement techniques and technologies. For example, the meter was originally defined as one ten-millionth of the distance from the North Pole to the equator, but is now defined as the distance light travels in a vacuum in 1/299,792,458 of a second.
Yes, there are ongoing discussions and proposals for changes to the current standards of measurement, particularly in the areas of mass and time. For example, there have been discussions about redefining the kilogram to be based on a fundamental constant of nature instead of a physical object, and redefining the second to be based on the natural frequency of a cesium atom rather than the Earth's rotation.