The way I understand it, the term *flat character* is used for a character that is **both** two-dimensional and has a flat character arc. Here, a *two-dimensional character* is character that shows a little, and not very complex, emotions and/or traits.

However, sometimes, I hear *two-dimensional character* explained *flat character*. Other times, I hear *flat character* explained as a character that has a flat arc, but one that's not necessarily two-dimensional.

From [this Britannica definition](https://www.britannica.com/art/flat-character), who defines the term in the first way, I suspect that maybe this is a case of these terms originally being well-defined, but after misuse, are now ambiguous?

**EDIT:**

I forgot to mention these further confuddling details. The Britannica definition cites E. M. Forster. That means that according to Britannica, E. M. Forster said a flat character is someone who is both two-dimensional and has a flat character arc. However, he also said that a flat character represents an idea, by being unchanging, almost like a force of nature. This feature however, only requires their arc being flat. In fact, in stories where the flat arc belongs the MC, they are often representative of an idea, yet whilst also being three-dimensional, naturally.

**EDIT 2:**

I think I have to be clearer. My question, and confusion, is caused by the contradictions of the multiple definitions. @Phillip's answer provides another definition, but doesn't explain why it is different from the two featured in my question: which definition, if any, is correct? What is the original definition? What definition is given by the most authoritative sources?