An attribution is an opinion about who was responsible for making a work of art.  Experts make attributions with differing degrees of certainty, depending on documentary evidence and judgments about style.  An attribution might be phrased as “attributed to X” (suggesting some degree of doubt), or “studio of X”, or “follower of X” or “imitator of X” (all indicating that in the expert’s opinion the work was not actually made by the master’s hand). See also: connoisseur.