This is a misunderstanding of the term: "pain text" sits opposite of "rich text", formats where the markup is part of the content. The terms are at a higher level of abstraction than just encoding, rich text being things like xml/html or word documents that require specialized parsers to retrieve the content. Plain text refers to files that don't have that kind of special markup language.
No, there is more than one valid definition, the one you have used is the one used e.g. by the Unicode standard (but their definition is narrowed to Unicode, so anything not Unicode isn't plain text). But it doesn't matter which definition of "text" somebody uses, encoding is always a problem if not explicitly set like it is possible when using XML and HTML.
Comments
This is a misunderstanding of the term: "pain text" sits opposite of "rich text", formats where the markup is part of the content. The terms are at a higher level of abstraction than just encoding, rich text being things like xml/html or word documents that require specialized parsers to retrieve the content. Plain text refers to files that don't have that kind of special markup language.
No, there is more than one valid definition, the one you have used is the one used e.g. by the Unicode standard (but their definition is narrowed to Unicode, so anything not Unicode isn't plain text). But it doesn't matter which definition of "text" somebody uses, encoding is always a problem if not explicitly set like it is possible when using XML and HTML.
Love that typo :D