I understand that essentially a word is "archaic" if it is old and not really used much today. What I'm interested in is if there is something quantifiable that makes a word archaic or not. For example,
- Must it be at least so many years old (e.g. I wouldn't call "golly" archaic, but I would call it old and out of use)?
- Must it currently not have a use that is superseded by a different word (e.g. "fortnight" always seemed archaic to me, but there is no modern word that replaces it, which might be why dictionaries do not list it as such)?
- Apparently the replacement word does not have to be newer (e.g. "to" predates "unto"), which certainly confuses things.
What are the quantifiable measures that make a word "archaic" and according to what authority?