A panegyric was a speech in praise of the Emperor. Panegyrics were considered something of an art form showing off the speaker's skill in oratory. As Disraeli once remarked, "Everyone likes flattery, and when you come to royalty, you should lay it on with a trowel." However, as the Bryn Mawr Classical Review 2003.07.37 makes clear, there was plenty of room for subtle differentiations.
The earliest Roman panegyric still extant is a speech by Pliny the Younger in praise of the emperor Trajan. Latin original. No English version is available on the internet that I can find.
It was under Domitian and Constantine that panegyric came into its own, however, and Gibbon draws on a collection of 12 panegyrics (known as panegyrici veteres latini) delivered before those emperors as one of his sources. He obviously finds the panegyrics more useful to the historian than does the compiler of the Dictionary of Greek and Roman Mythology and Biography here (scroll down to the bottom):
Discourses of this description must for the most part be as devoid of all sincerity and truth as they are, from their very nature, destitute of all genuine feeling or passion, and hence, at best, resolve themselves into a mere cold display of artistic dexterity, where the attention of the audience is kept alive by a succession of epigrammatic points, carefully balanced antitheses, elaborate metaphors, and well-timed cadences, where the manner is everything, the matter nothing. To look to such sources for historical information is obviously absurd. Success would in every case be grossly exaggerated, defeat carefully concealed, or interpreted to mean victory. The friends and allies of the sovereign would be daubed with fulsome praise, his enemies overwhelmed by a load of the foulest calumnies. We cannot learn what the course of events really was, but merely under what aspect the ruling powers desired that those events should be viewed, and frequently the misrepresentations are so flagrant that we are unable to detect even a vestige of truth lurking below.
Here is a blog entry on the development of the panegyric. [The main blog is difficult to isolate, it seems to be from a conglomerate of bloggers at the University of Oregon. IH]
Although admitting that strict adherence to the truth was not the panegyricists' strong point, the OCD agrees with Gibbon that they do contain information of use to the historian and at least show what message the emperor was keen to get across to the public.