Unix98 PTYs
BSD PTYs have been rendered obsolete by Unix98 ptys whose nomenclature does not limit the number of pseudo-terminals and access to which occurs without danger of races.
/dev/ptmx is the "pseudo-terminal master multiplexer". Opening it returns a file descriptor of a master node and causes an associated slave node /dev/pts/N node to be created.
Read more about this topic: Pseudo Terminal