| context | $\langle X,T\rangle$ … topological space |
| definiendum | $B\in$ it |
| inclusion | $B\subseteq T$ |
| for all | $U\in T$ |
| exists | $C\subseteq B$ |
| postulate | $U=\bigcup C$ |
A base $B$ for a topology is a collection its open sets, which suffice to cover any open set.