Plain text is a type of content comprised solely of readable characters, without formatting or metadata. It is the simplest format available for electronic representation of words, sentences, and messages. Formatting, delineation, and other features can be simulated textually using punctuation in ways other than in normal writing, such as commas (,) or pipes (|) to separate columns, brackets to mark special content like labels, and surrounding words to indicate things like *emphasis* or /italics/ or _underline_. Many standards or conventions exist for these things, but those standards each have their own name and definition above and beyond just the text.
Contrast to rich text that includes formatting such as bold, italics, hyperlinks, lists, etc.
Contrast to multimedia content where text might be mixed with images or audio content.
Contrast to markup where content is provided as text with well defined rules for special behavior like _underline_ or <pre>tags</pre>, which produces rich text or even multimedia when processed and displayed.