Definition:Unit


 * 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.