\begin{definition}[Definition:Collation]
A '''collation''' is a structured alignment with certain placeholders that underpins the construction of formal languages.
These placeholders may be replaced by elements of an alphabet $\AA$ under consideration.
A '''collation in $\AA$''' is one where all placeholders are replaced by symbols from $\AA$.
For example, if we take $\square$ to denote a placeholder, then $\square\square\square\square\square$ represents the collation "a word of length $5$".
We can see that then the word "sheep" is an instance of the '''collation''' "a word of length $5$" '''in the English alphabet''', as is "axiom".
Typical examples of '''collations''' encountered in mathematics are words or structured graphics like labeled trees.
\end{definition}