In the mathematical field of order theory an order isomorphism is a special kind of monotone function that constitutes a suitable notion of isomorphism for partially ordered sets (posets). Whenever two posets are order isomorphic, they can be considered to be "essentially the same" in the sense that one of the orders can be obtained from the other just by renaming of elements. Two strictly weaker notions that relate to order isomorphisms are order embeddings and Galois connections.
Formally, given two posets and , an order isomorphism from to is a bijective function from to with the property that, for every and in , if and only if . That is, it is a bijective order-embedding.
It is also possible to define an order isomorphism to be a surjective order-embedding. The two assumptions that cover all the elements of and that it preserve orderings, are enough to ensure that is also one-to-one, for if then (by the assumption that preserves the order) it would follow that and , implying by the definition of a partial order that .
- The identity function on any partially ordered set is always an order automorphism.
- Negation is an order isomorphism from to (where is the set of real numbers and denotes the usual numerical comparison), since -x ≥ -y if and only if x ≤ y.
- The open interval (again, ordered numerically) does not have an order isomorphism to or from the closed interval : the closed interval has a least element, but the open interval does not, and order isomorphisms must preserve the existence of least elements.
 Order types
If is an order isomorphism, then so is its inverse function. Also, if is an order isomorphism from to and is an order isomorphism from to , then the function composition of and is itself an order isomorphism, from to .
Two partially ordered sets are said to be order isomorphic when there exists an order isomorphism from one to the other. Identity functions, function inverses, and compositions of functions correspond, respectively, to the three defining characteristics of an equivalence relation: reflexivity, symmetry, and transitivity. Therefore, order isomorphism is an equivalence relation. The class of partially ordered sets can be partitioned by it into equivalence classes, families of partial orders that are all isomorphic to each other. These equivalence classes are called order types.
- Block (2011); Ciesielski (1997).
- This is the definition used by Ciesielski (1997). For Bloch (2011) and Schröder (2003) it is a consequence of a different definition.
- This is the definition used by Bloch (2011) and Schröder (2003).
- Schröder (2003), p. 13.
- See example 4 of Ciesielski (1997), p. 39., for a similar example with integers in place of real numbers.
- Ciesielski (1997), example 1, p. 39.
- Ciesielski (1997); Schröder (2003).
- Ciesielski (1997).
- Bloch, Ethan D. (2011), Proofs and Fundamentals: A First Course in Abstract Mathematics, Undergraduate Texts in Mathematics (2nd ed.), Springer, pp. 276–277, ISBN 9781441971265.
- Ciesielski, Krzysztof (1997), Set Theory for the Working Mathematician, London Mathematical Society Student Texts 39, Cambridge University Press, pp. 38–39, ISBN 9780521594653.
- Schröder, Bernd Siegfried Walter (2003), Ordered Sets: An Introduction, Springer, p. 11, ISBN 9780817641283.