Unit

The word unit can mean more than one thing:

• One, the first natural number.
• The natural or usual or smallest measure of something, of which there are multiples and of which there may be fractions.
• In most physical contexts, for example science and engineering, 'unit' is used to mean a physical unit of measurement of a quantity. Systems of units are formed which are typically characterized by the units chosen for mass, distance, and time. Such systems of units include
  • SI (previously known as the Metre-Kilogram-Second system of units (MKS)), the most commonly used units worldwide
  • Centimeter-Gram-Second (CGS) system of units, derived from SI and also commonly used worldwide; also one of two standard unit systems in astronomy
  • Metre-Tonne-Second system of units (MTS), little used
  • Imperial units, also known as British units, were historically used in Britain and its Commonwealth, and are similar (but not identical) to the current day US customary units
  • U.S. customary units, primarily used in the USA
  • Chinese system of units, used primarily in China
• A currency is a unit of money (a monetary unit).
• In mathematics, "unit" often refers to the number one in some sense:
  • A unit vector is a vector with length 1.
  • The unit circle is the cirle with radius 1 centered at the origin of the coordinate system.
  • The unit interval is the interval of all real numbers between 0 and 1.
  • The imaginary unit of complex numbers is the number i, a number whose square is -1.
  • A root of unity is a complex number a power of which is 1.
  • A unit of a ring is an element of the ring that is invertible with respect to the ring multiplication.
• Military units:
  • Unit 101 was an Israeli special operations unit.
  • Unit 731 was a secret military unit of the Japanese army.
• Functional units are components of a system that perform a specific function:
  • The Central processing unit (CPU) is the processor of a computer; Arithmetic and Logical Unit, Control unit, Memory management unit and Floating Point Unit are important components of the CPU.
• A multiple unit is a passenger train whose carriages have their own motors.
• In the science fiction television series, "Doctor Who", the initials UNIT stand for the United Nations Intelligence Taskforce.

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