Plain Text could mean...
The opposite of CypherText? where the text is not encrypted.
This would be more appropriately called ClearText?. ROT13 is a simple encoding scheme, but the resulting CypherText? looks much like Plain Text.
Text that has no SpecialFormatting.
Plain Text usually means 7-bit AsciiCode, despite all attempts at expanding this woefully inadequate encoding scheme into something at least capable of spelling English, which has one of the smallest alphabets of all modern languages, correctly. If you think ASCII is able to spell all English words, you are somewhat naïve about English orthography and it would be nice if you would coöperate and buy a good dictionary.
The irony is that all of these extended ASCII encoding schemes were mutually incompatible, driving programmers and end-users to the lowest common denominator, plain 7-bit ASCII. UniCode in all its translation formats, especially UtfEight, promises a way out of this morass, but I'm not holding my breath.