Attribution
It is not always known who painted a picture. An attribution is an assessment of who was responsible for creating a particular work.
Attributions are made with different degrees of certainty, depending on factors such as style and documentary and scientific evidence:
'by Raphael' shows reasonable certainty about the attribution
'attributed to Raphael' intimates a degree of doubt about the authorship of the picture
'by the Studio of Raphael' means painted by a pupil of the named artist, probably under his direction
'a Follower of Raphael' is someone who admired the artist's style, but was not necessarily a pupil of his
'an Imitator of Raphael' is one who slavishly admired the artist, but may have worked at a much later date
I found this list of attribution qualifiers from the National Gallery in London.
Evidently there is an entire academic discipline dedicated to weasel words ;-)
I found some lists that were much longer!
DLB