[munk-a]

Isn't it the other way? Plain text is markdown but markdown isn't plain text?

You're leveraging some semi-readable things[1] to inject into the document to have special meaning, but normally written[2] text will render just fine in markdown.

1. I say semi-readable because BS like image and link tags really stretch the bounds of "as someone would naturally write it".

2. Normally written here is carrying a fair amount of weight but the "obscure" symbols used in markdown (that aren't just plain text conventions anyways) are extremely uncommon in normal communication.

[Tagbert]

Markdown is a specific structured subset of plain text. Plain text is the larger set containing Markdown.

That means that Markdown is plain text but plain text may not be Markdown.