Kilo is a decimal unit prefix in the ^{3}). It is used in the

^{10} bytes (= 1024 bytes), because of the mathematical coincidence that 2^{10} is approximately 10^{3}. The reason for this application is that digital hardware and architectures natively use base 2 exponentiation, and not decimal systems. JEDEC memory standards still permit this definition, but acknowledge the correct SI usage.
NIST comments on the confusion caused by these contrasting definitions: "Faced with this reality, the ''IEEE Standards Board'' decided that IEEE standards will use the conventional, internationally adopted, definitions of the SI prefixes", instead of kilo for 1024.Definition of binary prefixes at NIST

/ref> To address this conflict, a new set of binary prefixes has been introduced, which is based on powers of 2. Therefore, 1024 bytes are defined as one

^{2} means one square kilometre or the area of a square that measures 1000 m on each side or 10^{6} m^{2} (as opposed to 1000 square meters, which is the area of a square that measures 31.6 m on each side).
* 1 km^{3} means one cubic kilometre or the volume of a cube that measures 1000 m on each side or 10^{9} m^{3} (as opposed to 1000 cubic meters, which is the volume of a cube that measures 10 m on each side).

metric system
The metric system is a system of measurement that succeeded the decimalised system based on the metre that had been introduced in France in the 1790s. The historical development of these systems culminated in the definition of the Interna ...

denoting multiplication by one thousand
1000 or one thousand is the natural number following 999 and preceding 1001. In most English-speaking countries, it can be written with or without a comma or sometimes a period separating the thousands digit: 1,000.
A group of one thousand ...

(10International System of Units
The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. E ...

, where it has the symbol k, in lowercase
Letter case is the distinction between the letters that are in larger uppercase or capitals (or more formally ''majuscule'') and smaller lowercase (or more formally ''minuscule'') in the written representation of certain languages. The writin ...

.
The prefix ''kilo'' is derived from the Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
* Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancesto ...

word (), meaning "thousand".
In 19th century English it was sometimes spelled chilio, in line with a puristic opinion by Thomas Young. As an opponent of suggestions to introduce the metric system in Britain, he qualified the nomenclature adopted in France as barbarous.
Examples

* one kilogram (kg) is 1000gram
The gram (originally gramme; SI unit symbol g) is a unit of mass in the International System of Units (SI) equal to one one thousandth of a kilogram.
Originally defined as of 1795 as "the absolute weight of a volume of pure water equal to th ...

s
* one kilometre (km) is 1000 metres
* one kilojoule
The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applie ...

(kJ) is 1000 joule
The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force appli ...

s
* one kilolitre (kL) is 1000 litre
The litre (international spelling) or liter (American English spelling) (SI symbols L and l, other symbol used: ℓ) is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metre (m3). ...

s
* one kilobaud (kBd) is 1000 baud
* one kilohertz (kHz) is 1000 hertz
* one kilobit (kb) is 1000 bits
* one kilobyte (kB) is 1000 bytes
* one kiloohm is (kΩ) is 1000 ohms
* one kilosecond (ks) is 1000 seconds
*one kilotonne (kt) is 1000 tonnes
By extension, currencies are also sometimes preceded by the prefix kilo-:
* one kiloeuro (k€) is 1000 euro
The euro (symbol: €; code: EUR) is the official currency of 19 out of the member states of the European Union (EU). This group of states is known as the eurozone or, officially, the euro area, and includes about 340 million citizens . ...

s
* one kilodollar (k$) is 1000 dollar
Dollar is the name of more than 20 currencies. They include the Australian dollar, Brunei dollar, Canadian dollar, Hong Kong dollar, Jamaican dollar, Liberian dollar, Namibian dollar, New Taiwan dollar, New Zealand dollar, Singapore dollar, ...

s
kilobyte

For the kilobyte, a second definition has been in common use in some fields ofcomputer science
Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to practical disciplines (including ...

and information technology. It uses ''kilobyte'' to mean 2/ref> To address this conflict, a new set of binary prefixes has been introduced, which is based on powers of 2. Therefore, 1024 bytes are defined as one

kibibyte
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable un ...

(1 KiB).
Exponentiation

When units occur in exponentiation, such as in square and cubic forms, any multiplier prefix is considered part of the unit, and thus included in the exponentiation. * 1 kmSee also

*milli-
''Milli'' (symbol m) is a unit prefix in the metric system denoting a factor of one thousandth (10−3). Proposed in 1793, and adopted in 1795, the prefix comes from the Latin , meaning ''one thousand'' (the Latin plural is ). Since 1960, the p ...

(inverse of kilo- prefix, denoting a factor of 1/1000)
* kibi- (binary prefix, denoting a factor of 1024)
* RKM code
The RKM code, also referred to as "letter and numeral code for resistance and capacitance values and tolerances", "letter and digit code for resistance and capacitance values and tolerances", or informally as "R notation" is a notation to specif ...

References

{{SI prefixes (infobox) SI prefixes 1000 (number) he:תחיליות במערכת היחידות הבינלאומית#קילו