Optional parameters. whose default values are specified further along the function/method entry.
I believe I first saw them used in Perl docs. Or was it PHP Manual? I forget. The use of square brackets to signify optional parameters have mostly become "invisible"/"transparent" to me, so I wouldn't be able to list down all language documentations following the same standard without having to browse through all of them.
However, not all languages support the "assigning by name" for optional function arguments as python, even though the documentation style is similar.