Afternoon Tea - well, Sherry, really
The occasion in the University's eating and drinking cycle where a previous Bursar offered Rincewind strong drink to steady his nerves, and stop him burbling on about bedbugs and gargoyles fleeing from the University to evade the attentions of a Sourcerer.
No evidence has yet been gleaned as to whether this is different to, or the same thing as, the Afternoon Tea referenced in The Science of Discworld III: Darwin's Watch, where Mustrum Ridcully temporarily abolishes in order to focus the minds of the wizards on a job that cannot wait. It is of course possible, as a long time seperates both books, that at the time of Sourcery, the University's eating and drinking cycle was still evolving into the magnificently complex and baroque thing it has become by The Science of Discworld III: Darwin's Watch and Unseen Academicals..