Definition:Unit


 * Arithmetic and so on:
 * Numbers: An instance of the number $1$ (one).


 * Abstract Algebra:
 * Unit of Ring


 * Set Systems:
 * Unit of System of Sets.


 * Algebras:
 * Unit of a unitary algebra

The concepts are related: it is apparent that the unit of an algebra of sets $\SS$ is the unity of $\SS$ considered as a ring $\struct {\SS, *, \cap}$.

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

Also see

 * Definition:Unity