Since nobody seems to have answered this question, and I'm the one who uses it the most, an arc is basically a plot thread within a larger story. Ah My Goddess is basically the story of Keiichi and Belldandy and their life together, but sometimes a group of chapters will tell a side story or just highlight a certain event. The main story is still going on, but these chapters, while still part of the overall story, form a story arc.
Examples include the mini-ninjas, Urd being possessed by the Lord of Terror, Skuld falling in love with Sentaro and getting her angel, Banpei falling in love with Sigil, etc.
Dark Horse tried to have one or two of these arcs per TPB (trade paperback), plus a couple of "stand alone" stories. This lead to their numbering system being off, which is why if you bought the Sora Unchain TPB, it was labled as Volume 19/20. It just isn't consistant with how it is published in Japan. I'm also fairly sure they have some of the stories out of place, but I'd have to check to be sure (I know for a fact that they cut out some stories from the first TPB, then went back and released 3 TPBs to cover what they had missed).
Incompitence or devious marketing ploy, you be the judge.
Larry
19-Feb-2005