The First Journey of Agatha Heterodyne is the full, official title of the original set of thirteen volumes in the story of Agatha Heterodyne, which has the blanket title Girl Genius. This part of the story was originally referred to as the "first half of the story" or "season one," as the Foglios discussed their plans to eventually create "a second half of the story" or "season two." For a long time it wasn't clear when, or even if, "season two" would start, but it did on At that point, the Foglios had decided to use the name Act 2 instead of "season two" for this part of the story, calling the first part Act 1.
Eventually, they decided to use the name The Second Journey of Agatha Heterodyne, instead of Act 2, at least for official and marketing purposes, and retroactively renamed Act 1 as The First Journey of Agatha Heterodyne. However, Act 1 is still used as a much shorter nickname by both fans and the Foglios for the first part of Girl Genius. It seems likely that this new overall title will be used on the covers and title pages in future reprints of the first thirteen volumes, to match what has been done with The Second Journey of Agatha Heterodyne in the printed volumes of Act 2 so far.