<file system> An element of data storage in a {file system}.

	The history of computing is rich in varied kinds of files and
	{file systems}, whether ornate (e.g., {Macintosh file system}
	for a well-known case) or deficient (e.g., many simple
	pre-1980s file systems don't allow {directories}).

	However, the prototypical file has these characteristics:

	* It is a single sequence of bytes (but consider {Macintosh}
	{resource forks}).

	* It has a finite length, unlike, e.g. a {Unix} {device}.

	* It is stored in a {non-volatile storage} medium (but see
	{ramdrive}).

	* It exists (nominally) in a {directory}.

	* It has a name that it can be referred to by in file
	operations, possibly in combination with its {path}.

	Additionally, a file system may associate other information
	with a file, such as {permission} bits or other {file
	attributes}; timestamps for file creation, last revision, and
	last access; revision numbers (a` la VMS), and other kinds of
	{magic}.

	Compare: {document}.

	(1997-04-08)

1. <database> A value used to identify a {record} in a
	database, derived by applying some fixed function to the
	record.  The key is often simply one of the {fields} (a
	{column} if the database is considered as a table with records
	being rows, see "{key field}").  Alternatively the key may be
	obtained by applying some function, e.g. a {hash function}, to
	one or more of the fields.  The set of keys for all records
	forms an {index}.  Multiple indexes may be built for one
	database depending on how it is to be searched.

	2. <cryptography> A value which must be fed into the
	{algorithm} used to decode an encrypted message in order to
	reproduce the original {plain text}.  Some encryption schemes
	use the same (secret) key to encrypt and decrypt a message,
	but {public key encryption} uses a "private" (secret) key and
	a "public" key which is known by all parties.

	3. <hardware> An electromechanical {keyboard} button.

	(2003-07-04)

<mathematics> The area of theoretical computer science
	concerning what problems can be solved by any computer.

	A function is computable if an {algorithm} can be implemented
	which will give the correct output for any valid input.

	Since computer programs are {countable} but {real numbers} are
	not, it follows that there must exist real numbers that
	cannot be calculated by any program.  Unfortunately, by
	definition, there isn't an easy way of describing any of them!

	In fact, there are many tasks (not just calculating real
	numbers) that computers cannot perform.  The most well-known
	is the {halting problem}, the {busy beaver} problem is less
	famous but just as fascinating.

	["Computability", N.J. Cutland. (A well written
	undergraduate-level introduction to the subject)].

	["The Turing Omnibus", A.K. Dewdeney].

	(1995-01-13)

<data> To read data that is not in the native format of the
	application.  For example, a {web browser} will have its own
	way of storing {bookmarks} but it will usually provide a
	function to import bookmarks from {Internet Explorer}.  The
	alternative is to provide an independent external conversion
	utility but this is usually less convenient for the user.

	(2004-11-15)

1. <operating system> The {Unix} {superuser} account (with
	user name "root" and user ID 0) that overrides file
	permissions.  The term {avatar} is also used.  By extension,
	the privileged system-maintenance login on any {operating
	system}.

	See {root mode}, {go root}, {wheel}.

	[{Jargon File}]

	(1994-10-27)

	2. <operating system> {root directory}.

	(1996-11-21)

	3. <data> {root node}.

	(1998-11-14)