An integer that identifies an open file within a process. This number is obtained as a result of opening a file. Operations which read, write, or close a file would take the file descriptor as an input parameter.
In many operating system implementations, file descriptors are small integers which index a table of open files. In UNIX, file descriptors 0, 1, and 2 correspond to the standard input, standard output, and standard error files respectively. See also data dictionaries and XFD file.