Definition: A centimeter (symbol: cm) is a unit of length in the International System of Units (SI), the current form of the metric system. It is defined as 1/100 meters.

History/origin: A centimeter is based on the SI unit meter, and as the prefix "centi" indicates, is equal to one hundredth of a meter. Metric prefixes range from factors of 10^-18 to 10¹⁸based on a decimal system, with the base (in this case the meter) having no prefix and having a factor of 1. Learning some of the more commonly used metric prefixes, such as kilo-, mega-, giga-, tera-, centi-, milli-, micro-, and nano-, can be helpful for quickly navigating metric units

Current use: The centimeter, like the meter, is used in all sorts of applications worldwide (in countries that have undergone metrication) in instances where a smaller denomination of the meter is required. Height is commonly measured in centimeters outside of countries like the United States.

Definition: An inch (symbol: in) is a unit of length in the imperial and US customary systems of measurement. An inch was defined to be equivalent to exactly 25.4 millimeters in 1959. There are 12 inches in a foot and 36 inches in a yard.

History/origin: The term "inch" was derived from the Latin unit "uncia" which equated to "one-twelfth" of a Roman foot.
There have been a number of different standards for the inch in the past, with the current definition being based on the international yard. One of the earliest definitions of the inch was based on barleycorns, where an inch was equal to the length of three grains of dry, round barley placed end-to-end. Another version of the inch is also believed to have been derived from the width of a human thumb, where the length was obtained from averaging the width of three thumbs: a small, a medium, and a large one.

Current use: The inch is mostly used in the United States, Canada, and the United Kingdom. It is also sometimes used in Japan (as well as other countries) in relation to electronic parts, like the size of display screens.