In mathematics, a Hurwitz quaternion (or Hurwitz integer) is a quaternion whose components are either all integers or all half-integers (halves of odd integers; a mixture of integers and half-integers is excluded). The set of all Hurwitz quaternions is

${\displaystyle H=\left\{a+bi+cj+dk\in \mathbb {H} \mid a,b,c,d\in \mathbb {Z} \;{\mbox{ or }}\,a,b,c,d\in \mathbb {Z} +{\tfrac {1}{2}}\right\}.}$

That is, either a, b, c, d are all integers, or they are all half-integers. H is closed under quaternion multiplication and addition, which makes it a subring of the ring of all quaternions H. Hurwitz quaternions were introduced by Adolf Hurwitz (1919).

A Lipschitz quaternion (or Lipschitz integer) is a quaternion whose components are all integers. The set of all Lipschitz quaternions

${\displaystyle L=\left\{a+bi+cj+dk\in \mathbb {H} \mid a,b,c,d\in \mathbb {Z} \right\}}$

forms a subring of the Hurwitz quaternions H. Hurwitz integers have the advantage over Lipschitz integers that it is possible to perform Euclidean division on them, obtaining a small remainder.

Both the Hurwitz and Lipschitz quaternions are examples of noncommutative domains which are not division rings.