The Collaborative International Dictionary of English v.0.48:

arugula \arugula\ n.
   1. an erect European annual (Eruca vesicaria sativa) of the
      mustard family, often grown as a salad crop to be
      harvested when young and tender.

   Syn: rocket, roquette, garden rocket, rocket salad, Eruca
        sativa.
        [WordNet 1.5]