In algebra, a **unit** or **invertible element**^{[a]} of a ring is an invertible element for the multiplication of the ring. That is, an element u of a ring R is a unit if there exists v in R such that

where 1 is the multiplicative identity; the element v is unique for this property and is called the multiplicative inverse of u.

Less commonly, the term *unit* is sometimes used to refer to the element 1 of the ring, in expressions like *ring with a unit* or *unit ring*, and also unit matrix. Because of this ambiguity, 1 is more commonly called the "unity" or the "identity" of the ring, and the phrases "ring with unity" or a "ring with identity" may be used to emphasize that one is considering a ring instead of a rng.

The multiplicative identity 1 and its additive inverse −1 are always units. More generally, any root of unity in a ring R is a unit: if *r ^{n}* = 1, then

In the ring of integers **Z**, the only units are 1 and −1.

In the ring **Z**/*n***Z** of integers modulo n, the units are the congruence classes (mod *n*) represented by integers coprime to n. They constitute the multiplicative group of integers modulo n.

In the ring **Z**[√3] obtained by adjoining the quadratic integer √3 to **Z**, one has (2 + √3)(2 − √3) = 1, so 2 + √3 is a unit, and so are its powers, so **Z**[√3] has infinitely many units.

More generally, for the ring of integers R in a number field F, Dirichlet's unit theorem states that *R*^{×} is isomorphic to the group

where is the (finite, cyclic) group of roots of unity in R and n, the rank of the unit group, is

where are the number of real embeddings and the number of pairs of complex embeddings of F, respectively.

This recovers the **Z**[√3] example: The unit group of (the ring of integers of) a real quadratic field is infinite of rank 1, since .

For a commutative ring R, the units of the polynomial ring *R*[*x*] are the polynomials

such that is a unit in R and the remaining coefficients are nilpotent, i.e., satisfy for some

such that is a unit in R.

The unit group of the ring M_{n}(*R*) of *n* × *n* matrices over a ring R is the group GL_{n}(*R*) of invertible matrices. For a commutative ring R, an element A of M_{n}(*R*) is invertible if and only if the determinant of A is invertible in R. In that case, *A*^{−1} can be given explicitly in terms of the adjugate matrix.

For elements x and y in a ring R, if is invertible, then is invertible with inverse ;^{[6]} this formula can be guessed, but not proved, by the following calculation in a ring of noncommutative power series:

See Hua's identity for similar results.

A commutative ring is a local ring if *R* − *R*^{×} is a maximal ideal.

As it turns out, if *R* − *R*^{×} is an ideal, then it is necessarily a maximal ideal and *R* is local since a maximal ideal is disjoint from *R*^{×}.

If R is a finite field, then *R*^{×} is a cyclic group of order .

Every ring homomorphism *f* : *R* → *S* induces a group homomorphism *R*^{×} → *S*^{×}, since f maps units to units. In fact, the formation of the unit group defines a functor from the category of rings to the category of groups. This functor has a left adjoint which is the integral group ring construction.^{[7]}

The group scheme is isomorphic to the multiplicative group scheme over any base, so for any commutative ring R, the groups and are canonically isomorphic to . Note that the functor (that is, ) is representable in the sense: for commutative rings R (this for instance follows from the aforementioned adjoint relation with the group ring construction). Explicitly this means that there is a natural bijection between the set of the ring homomorphisms and the set of unit elements of R (in contrast, represents the additive group , the forgetful functor from the category of commutative rings to the category of abelian groups).

Suppose that R is commutative. Elements r and s of R are called *associate* if there exists a unit u in R such that *r* = *us*; then write *r* ∼ *s*. In any ring, pairs of additive inverse elements^{[c]} *x* and −*x* are associate. For example, 6 and −6 are associate in **Z**. In general, ~ is an equivalence relation on R.

Associatedness can also be described in terms of the action of *R*^{×} on R via multiplication: Two elements of R are associate if they are in the same *R*^{×}-orbit.

In an integral domain, the set of associates of a given nonzero element has the same cardinality as *R*^{×}.

The equivalence relation ~ can be viewed as any one of Green's semigroup relations specialized to the multiplicative semigroup of a commutative ring R.