<s>
In	O
the	O
Standard	B-Language
Generalized	I-Language
Markup	I-Language
Language	I-Language
(	O
SGML	B-Language
)	O
,	O
an	O
entity	O
is	O
a	O
primitive	O
data	O
type	O
,	O
which	O
associates	O
a	O
string	O
with	O
either	O
a	O
unique	O
alias	O
(	O
such	O
as	O
a	O
user-specified	O
name	O
)	O
or	O
an	O
SGML	B-Language
reserved	O
word	O
(	O
such	O
as	O
#DEFAULT	O
)	O
.	O
</s>
<s>
Entities	O
are	O
foundational	O
to	O
the	O
organizational	O
structure	O
and	O
definition	O
of	O
SGML	B-Language
documents	O
.	O
</s>
<s>
The	O
SGML	B-Language
specification	O
defines	O
numerous	O
entity	O
types	O
,	O
which	O
are	O
distinguished	O
by	O
keyword	O
qualifiers	O
and	O
context	O
.	O
</s>
<s>
An	O
entity	O
string	O
value	O
may	O
variously	O
consist	O
of	O
plain	O
text	O
,	O
SGML	B-Language
tags	O
,	O
and/or	O
references	O
to	O
previously	O
defined	O
entities	O
.	O
</s>
<s>
A	O
parameter	O
entity	O
can	O
only	O
be	O
referenced	O
within	O
the	O
document	B-Language
type	I-Language
definition	I-Language
(	O
DTD	O
)	O
.	O
</s>
<s>
An	O
entity	O
declaration	O
may	O
have	O
a	O
literal	O
value	O
,	O
or	O
may	O
have	O
some	O
combination	O
of	O
an	O
optional	O
SYSTEM	O
identifier	O
,	O
which	O
allows	O
SGML	B-Language
parsers	O
to	O
process	O
an	O
entity	O
's	O
string	O
referent	O
as	O
a	O
resource	O
identifier	O
,	O
and	O
an	O
optional	O
PUBLIC	O
identifier	O
,	O
which	O
identifies	O
the	O
entity	O
independent	O
of	O
any	O
particular	O
representation	O
.	O
</s>
<s>
In	O
XML	B-Protocol
,	O
a	O
subset	O
of	O
SGML	B-Language
,	O
an	O
entity	O
declaration	O
may	O
not	O
have	O
a	O
PUBLIC	O
identifier	O
without	O
a	O
SYSTEM	O
identifier	O
.	O
</s>
<s>
When	O
an	O
external	O
entity	O
references	O
a	O
complete	O
SGML	B-Language
document	O
,	O
it	O
is	O
known	O
in	O
the	O
calling	O
document	O
as	O
an	O
SGML	B-Language
document	O
entity	O
.	O
</s>
<s>
An	O
SGML	B-Language
document	O
is	O
a	O
text	O
document	O
with	O
SGML	B-Language
markup	O
defined	O
in	O
an	O
SGML	B-Language
prologue	O
(	O
i.e.	O
,	O
the	O
DTD	O
and	O
subdocuments	O
)	O
.	O
</s>
<s>
A	O
complete	O
SGML	B-Language
document	O
comprises	O
not	O
only	O
the	O
document	O
instance	O
itself	O
,	O
but	O
also	O
the	O
prologue	O
and	O
,	O
optionally	O
,	O
the	O
SGML	B-Language
declaration	O
(	O
which	O
defines	O
the	O
document	O
's	O
markup	O
syntax	O
and	O
declares	O
the	O
character	B-General_Concept
encoding	I-General_Concept
)	O
.	O
</s>
<s>
An	O
entity	O
is	O
defined	O
via	O
an	O
entity	O
declaration	O
in	O
a	O
document	O
's	O
document	B-Language
type	I-Language
definition	I-Language
(	O
DTD	O
)	O
.	O
</s>
<s>
An	O
external	O
general	O
entity	O
named	O
greeting2	O
exists	O
and	O
consists	O
of	O
the	O
text	O
found	O
in	O
the	O
resource	O
identified	O
by	O
the	O
URI	B-Protocol
file:///hello.txt	O
.	O
</s>
<s>
Names	O
for	O
entities	O
must	O
follow	O
the	O
rules	O
for	O
SGML	B-Language
names	I-Language
,	O
and	O
there	O
are	O
limitations	O
on	O
where	O
entities	O
can	O
be	O
referenced	O
.	O
</s>
<s>
Standard	O
entity	O
sets	O
for	O
SGML	B-Language
and	O
some	O
of	O
its	O
derivatives	O
have	O
been	O
developed	O
as	O
mnemonic	O
devices	O
,	O
to	O
ease	O
document	O
authoring	O
when	O
there	O
is	O
a	O
need	O
to	O
use	O
characters	O
that	O
are	O
not	O
easily	O
typed	O
or	O
that	O
are	O
not	O
widely	O
supported	O
by	O
legacy	O
character	B-General_Concept
encodings	I-General_Concept
.	O
</s>
<s>
Each	O
such	O
entity	O
consists	O
of	O
just	O
one	O
character	O
from	O
the	O
Universal	O
Character	B-General_Concept
Set	I-General_Concept
.	O
</s>
<s>
Although	O
any	O
character	O
can	O
be	O
referenced	O
using	O
a	O
numeric	B-General_Concept
character	I-General_Concept
reference	I-General_Concept
,	O
a	O
character	B-General_Concept
entity	I-General_Concept
reference	I-General_Concept
allows	O
characters	O
to	O
be	O
referenced	O
by	O
name	O
instead	O
of	O
code	O
point	O
.	O
</s>
<s>
For	O
example	O
,	O
HTML	O
4	O
has	O
252	O
built-in	O
character	B-General_Concept
entities	I-General_Concept
that	O
do	O
not	O
need	O
to	O
be	O
explicitly	O
declared	O
,	O
while	O
XML	B-Protocol
has	O
five	O
.	O
</s>
<s>
XHTML	B-Language
has	O
the	O
same	O
five	O
as	O
XML	B-Protocol
,	O
but	O
if	O
its	O
DTDs	O
are	O
explicitly	O
used	O
,	O
then	O
it	O
has	O
253	O
( 	O
 '	O
being	O
the	O
extra	O
entity	O
beyond	O
those	O
in	O
HTML	O
4	O
)	O
.	O
</s>
