Here's why:
* Archaic means a word is outdated or no longer in common use. It suggests the word belongs to a past era and is unlikely to be used in everyday speech or writing.
* Informal words are generally considered acceptable in casual settings, but they are not necessarily outdated.
* Formal words are often used in academic, professional, or literary contexts, and they are usually considered more polished and precise.
Therefore, an archaic word is not informal; it's simply no longer in common use. You might find an archaic word in historical literature or in specific academic contexts, but it wouldn't be appropriate for casual conversation or writing.
For example, "thee" and "thou" are archaic words. While they were once common, they are no longer used in everyday speech or writing. Instead, we use "you" in all situations.