Fossil word

A fossil word is a word that is broadly obsolete but remains in use due to its presence in an idiom or phrase. An example of a word is 'ado' in 'much ado'. An example of a phrase is 'in point' (relevant), which is found in the phrases 'case in point' (or 'case on point' in the legal context) and 'in point of fact', but is rarely used outside of a legal context.