Definition: A kilogram (symbol: kg) is the base unit of mass in the International System of Units (SI). It is currently defined as being equal to the mass of the International Prototype of the Kilogram (IPK), a cylinder made of a platinum-iridium alloy.
History/origin: The name kilogram was derived from the French “kilogramme,” which in turn came from adding Greek terminology meaning “a thousand,” before the Late Latin term “gramma” meaning “a small weight.”
Unlike the other SI base units, the kilogram is the only SI base unit with an SI prefix. SI is a system based on the meter-kilogram-second system of units rather than a centimeter-gram-second system. This is at least in part due to the inconsistencies and lack of coherence that can arise through use of centimeter-gram-second systems, such as those between the systems of electrostatic and electromagnetic units.
The kilogram was originally defined as the mass of one liter of water at its freezing point in 1794, but was eventually re-defined, since measuring the mass of a volume of water was imprecise and cumbersome. The current definition of a kilogram, defined as being equal to the mass of a physical prototype, is still imperfect. This is evidenced by the fact that the mass of the original prototype for the kilogram now weighs 50 micrograms less than other copies of the standard kilogram.
Definition: A pound (symbol: lb) is a unit of mass used in the imperial and US customary systems of measurement. The international avoirdupois pound (the common pound used today) is defined as exactly 0.45359237 kilograms. The avoirdupois pound is equivalent to 16 avoirdupois ounces.
History/origin: The pound descended from the Roman libra, and numerous different definitions of the pound were used throughout history prior to the international avoirdupois pound that is widely used today. The avoirdupois system is a system that was commonly used in the 13th century. It was updated to its current form in 1959. It is a system that was based on a physical standardized pound that used a prototype weight. This prototype weight could be divided into 16 ounces, a number that had three even divisors (8, 4, 2). This convenience could be the reason that the system was more popular than other systems of the time that used 10, 12, or 15 subdivisions.