Ben and I decided that we needed more strips and punchlines that only make sense to hardcore readers. Look forward to jokes so inaccessible even we, the authors, don't get them.
A continuity nod is a reference to some aspect in a series' past or an occurrence within the universe. The nod is not so much a major plot point, but more of a tilt of the hat to the fans that may know these little details. It is a sort of Shout-Out.
In remakes of older TV shows or movies, this may often manifest itself as a form of Stunt Casting. If characters reference something outside the universe itself but has special meaning some other way (such as an Alternate Continuity or adaptation), it's a Mythology Gag. If it's a joke about the actors of the parts it's Actor Allusion or Casting Gag. References towards something that's been written out (or Take Thats towards something that people wish were written out) are Discontinuity Nods. A nod that affects the plot in some way can be considered a Olafs Hammer.
Distinct from a Call Back, as it does not actually drive any plot development, or a Call Forward, as it isn't going to be a plot point later on. If a single scene presents an unusually large number of Continuity Nods, all at once, you may have fallen under a Continuity Cavalcade. If there are so many Continuity Nods they detract from the story itself, it's Continuity Porn.
Remember, it's only a continuity nod if the events happen in the same continuity. Otherwise it's a Mythology Gag.