Full Idea
Fallacies are errors in reasoning, labelled as 'formal' if a clear rule has been breached, and 'informal' if some less precise error has been made.
Gist of Idea
Fallacies are errors in reasoning, 'formal' if a clear rule is breached, and 'informal' if more general
Source
PG (Db (ideas) [2031])
Book Reference
A Reaction
Presumably there can be a grey area between the two.