Arvindr was the sword of an unknown dragon rider (possibly Thuviel) mentioned in Brisingr as a possible replacement sword for Eragon. However, Arvindr was in the city of Nädindel, which was too far away for Eragon to reach quickly. The color of Arvindr is purple, but the proportions are unknown.
The name is from Old Norse. It could have several translations, as the first part, ar, means several things. However, "vindr" is only translated as wind and "ari" means eagle, so "Wind-Eagle" is a probable translation.
The name could also come from the Old Norse name Arnvid/Arvid (or Arnvoir). Therefore, one might believe that the weapons' name might be translated to "forest eagle", as arn means eagle and vidh means forest.