If x is a list, then flatten x effectively removes the braces surrounding any elements of x that happen to be lists.

If x is a matrix, then flatten x puts the elements of x in a single row, ordered by column. If x is an $m\times n$ matrix, then flatten x is a $1\times mn$ matrix.



The matrix produced by flatten m is not the same as the matrix given by unnesting the list used to define the matrix.
The object flatten is a method function with a single argument.