Definition:Unit


 * Numbers: An instance of the number one.
 * Abstract Algebra: Unit of a Ring.
 * Set Systems: Unit of a System of Sets.

The concepts are related: it is apparent that the unit of an algebra of sets $\mathcal S$ is the unity of $\mathcal S$ considered as a ring $\left({\mathcal S, *, \cap}\right)$.

It may also refer to
 * Units of Measurement: A definite magnitude of a physical quantity which is agreed by convention.