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, 'unit' is used to mean a physical unit.
• 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.
• In science and engineering, 'unit' refers to the form of measurement of a quantity. These systems are typically characterized by the units chosen for mass, distance, and time. Unit systems include
  • SI (mks), the most commonly used units worldwide
  • cgs, derived from SI and also commonly used worldwide; also one of two standard unit systems in astronomy
  • imperial units (inlbs), also known as British units, were historically used in Britain and are similar (but not identical) to the current day
  • U.S. customary units, primarily used in the USA
• 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.