An interval is the subset of all the real numbers lying between a lower and an upper bound. If the interval includes a bound it is written with a square bracket, if not with a round bracket.
For example
\[ \begin{align*} x \in (l, u) &\rightarrow l < x < u\\ x \in [l, u) &\rightarrow l \leqslant x < u\\ x \in (l, u] &\rightarrow l < x \leqslant u\\ x \in [l, u] &\rightarrow l \leqslant x \leqslant u \end{align*} \]
where \(\in\) means within and \(\rightarrow\) means implies.