A common substring of a collection of strings is a substring of all the strings in the collection. For example, $\textrm{ACGT}$ is a common substring of $\textrm{TTGCAGG}\textbf{ACGT}\textrm{CCA}$ and $\textrm{CCAG}\textbf{ACGT}\textrm{TTAA}$. Note that $\textrm{ACGT}$ is not a longest common substring of these two strings, as $\textrm{GACGT}$ appears in both strings and is one symbol longer.