Sigh. I'm sorry for pushing on this. As Ned points out,
draft-ietf-822ext-mime-imb-03.txt does include a definition of the
term 'character set':
The term "character set" is used to refer to a method of converting a
"used" -> "used in MIME"
of octets into a sequence of characters. Note that unconditional and
unambiguous conversion in the other direction is not required, in that not
characters may be available in a given character set and a character set may
"available in" -> "representable by"
provide more than one sequence of octets to represent a particular character.
"a particular character" -> "a particular sequence of characters"
This definition is intended to allow various kinds of character encodings,
from simple single-table mappings such as US-ASCII to complex table switching
methods such as those that use ISO 2022's techniques. However, the
associated with a MIME character set name must fully specify the mapping to
performed from octets to characters. In particular, use of external
"performed from octets to characters" -> "performed"
information to determine the exact mapping is not permitted.
HISTORICAL NOTE: The term "character set" originated in the definition of
US-ASCII and similar 7bit and 8bit specifications. These define true sets.
However, the advent of multi-octet character encodings and switching
techniques have transformed character sets into entities that properly
speaking are no longer strictly sets. Some other communities have adopted
the term "character encoding" for what MIME calls a "character set" as a
This isn't a historical note, it's a compatibility note, or just NOTE.
US-ASCII isn't a 'true set' but it is a simple mapping from single
octets to single characters.
And the other encodings haven't transformed character sets but
have just caused some terminology changes in order to make
distinctions that didn't exist before.
NOTE: The term "character set" as used originally in MIME arose
with the use of US-ASCII and other 7bit and 8bit specifications
which employ a simple mapping from single octets to single
characters. The advent of multi-octet coded character sets and
switching techniques has made the situation more complex. For
example, some communities have adopted the term "character encoding"
for what MIME calls a "character set", while using the phrase "coded
character set" to denote an abstract mapping from integers (not
octets) to characters.