Thanks, Lad, I was thinking of its more ancient usage, like when St Robert Bellarmine and Cajetan talk about the Pope as a private person and seem to oppose to that when he is acting in a solemn capacity. I agree with you about its modern usage... perhaps that has always been the meaning, just curious to know if the subject has ever been treated of anywhere.
It may have been, but I don't know. I have generally seen explanations of papal infallibility and a list of the notes of infallibility, but don't recall any explanation for the term
ex cathedra. I think that in common parlance the term has become common shorthand for an "infallible" pronouncement, but my reading of Vatican I (and looking at the etymology), it's just one of the notes. He has to be teaching not only from the chair (in his official teaching capacity, as later known by the appearance of something in
Acta Apostolicae Sedis), but also has to be defining a matter of faith and morals that must be held by all the faithful.
Where it becomes tricky is that there are many times that the Pope seems to be defining a matter of faith and morals and insisting that the faithful accept the teaching, but without the solemn language. So, for instance, if you believe that Wojtyla was the pope, his teaching that only men could be ordained to the priesthood really does meet the notes of infallibility, even though it didn't appear in some formal dogmatic proclamation.
Here's Wojtyla from an "Apostolic Letter"
Ordinatio Sacerdotalis addressed to all the bishops:
Wherefore, in order that all doubt may be removed regarding a matter of great importance, a matter which pertains to the Church's divine constitution itself, in virtue of my ministry of confirming the brethren (cf. Lk 22:32) I declare that the Church has no authority whatsoever to confer priestly ordination on women and that this judgment is to be definitively held by all the Church's faithful.
1) "a matter which pertains to the Church's divine constitution" = matter of faith and morals
2) "in virtue of my ministry of confirming the brethren" = Pope acting as Pope, from Vatican I,
ex cathedra, that is as the teacher of all Christians
3) "I declare" = synonymous with define
4) "this judgment is to be definitively held by all the Church's faithful" = something to be held by all the faithful.
This meets all the notes of infallibility, since he was using nearly all the language of Vatican I.
Problem comes with the pre-Vatican I papal teaching, where they did not necessarily use the explicit language and one might have to INFER from the context whether he intended to teach as Pope and to definitively bind the faithful.