stdout is short for standard output. It is always descriptor number 1 in a unix program.
$ ls 1> /tmp/ls.stdout
This will redirect all of stdout to the file /tmp/ls.stdout
Standard output is defined in /usr/include/unistd.h as a macro that is called STDOUT_FILENO and its value is 1.
In BSD based OS' stdout can be represented as /dev/fd/1 or /dev/stdout. You can see more about this by typing "man 4 stdout" or "man 4 fd".