Kilo- is a unit prefix

SI prefix

The International System of Units specifies a set of unit prefixes known as SI prefixes or metric prefixes. An SI prefix is a name that precedes a basic unit of measure to indicate a decadic multiple or fraction of the unit. Each prefix has a unique symbol that is prepended to the unit symbol...

 in the metric system

Metric system

The metric system is an international decimalised system of measurement. France was first to adopt a metric system, in 1799, and a metric system is now the official system of measurement, used in almost every country in the world...

 denoting multiplication of the unit by one thousand.

For example:

• one kilogram
  Kilogram
  The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...
  
   is 1000 gram
  Gram
  The gram is a metric system unit of mass....
  
  s
• one kilometre
  Kilometre
  The kilometre is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in of a second...
  
   is 1000 metre
  Metre
  The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...
  
  s
• one kilojoule is 1000 joule
  Joule
  The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
  
  s
• one kilobaud
  Kilobaud
  Kilobaud may refer to:*One thousand bauds per second*P.H.I.R.M., a 1980s computer hacking group originally known as Kilobaud*Kilobaud Microcomputing, a homebrew computer magazine from the 1980s...
  
   is a rate of transfer used primarily in telecommunications
• one kilobyte
  Kilobyte
  The kilobyte is a multiple of the unit byte for digital information. Although the prefix kilo- means 1000, the term kilobyte and symbol KB have historically been used to refer to either 1024 bytes or 1000 bytes, dependent upon context, in the fields of computer science and information...
  
   is formally equal to 1000 byte
  Byte
  The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
  
  s. However, a second definition and usage has historically been in practice in many fields of computer science
  Computer science
  Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...
  
   and information technology, which defines the prefix kilo when used with byte or bit units of information as 1024 (2¹⁰); this is due to the mathematical coincidence that Thus, in these fields 1 kilobyte is equal to 1 kibibyte
  Kibibyte
  The kibibyte is a multiple of the unit byte for quantities of digital information. The binary prefix kibi means 1024; therefore, 1 kibibyte is . The unit symbol for the kibibyte is KiB. The unit was established by the International Electrotechnical Commission in 1999 and has been accepted for use...
  
  , a new unit standardized as part of the binary prefix
  Binary prefix
  In computing, a binary prefix is a specifier or mnemonic that is prepended to the units of digital information, the bit and the byte, to indicate multiplication by a power of 2...
  
  es to resolve the ambiguity.

The kilo prefix is derived from the Greek

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 word χίλιοι (chilioi), meaning thousand. It was originally adopted by Antoine Lavoisier

Antoine Lavoisier

Antoine-Laurent de Lavoisier , the "father of modern chemistry", was a French nobleman prominent in the histories of chemistry and biology...

's group in 1795, and introduced into the metric system in France with its establishment in 1799.

Exponentiation

When units occur in exponentiation

Exponentiation

Exponentiation is a mathematical operation, written as an, involving two numbers, the base a and the exponent n...

, such as in square and cubic forms, any multiplier prefix is considered part of the unit, and thus included in the exponentiation.

• 1 km² means one square kilometre or the area of a square
  Square (geometry)
  In geometry, a square is a regular quadrilateral. This means that it has four equal sides and four equal angles...
  
   of 1000 m by 1000 m or 10⁶ m².
• 1 km³ means one cubic kilometre or the volume of a cube
  Cube
  In geometry, a cube is a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex. The cube can also be called a regular hexahedron and is one of the five Platonic solids. It is a special kind of square prism, of rectangular parallelepiped and...
  
  of 1000 m by 1000 m by 1000 m or 10⁹ m³.