Definition:Interval/Ordered Set/Open

Definition
Let $(S, \preceq)$ be an ordered set.

Let $a, b \in S$.

Then $\left\{{ s \in S: (a \prec s) \land (s \prec b) }\right\}$ is called the open interval between $a$ and $b$.

It is written $(a \,.\,.\, b)$.

Also defined as
Some sources require that $a \preceq b$.