nLab
antisymmetric relation

A (binary) relation $\sim$ on a set $A$ is antisymmetric if any two elements that are related in both orders are equal:

$$\forall (x,y:A),x\sim y\wedge y\sim x\Rightarrow x=y$$\forall (x, y: A),\; x \sim y \;\wedge\; y \sim x \;\Rightarrow\; x = y

In the language of the $2$-poset-with-duals Rel of sets and relations, a relation $R:A\to A$ is antisymmetric if its intersection with its reverse is contained in the identity relation on $A$:

$$R\cap R^{\mathrm{op}}\subseteq {id}_A$$R \cap R^{op} \subseteq \id_A

If an antisymmetric relation is also reflexive (as most are in practice), then this containment becomes an equality.