I'm trying to find the correct term for a specific kind of totally ordered space:

Let $S$ be a totally ordered space with strict total order $<$.

Property: For any two $s_{1}$ and $s_{2}$ in $S$ where $s_1 < s_2$, there must exist some $s_{3}$ such that $s_{1} < s_{3}$ and $s_{3} < s_{2}$.

What is the name of this property? Thank you!