The word ** unit** can mean more than one thing:

- 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:
- 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.