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]

WordNet (r) 3.0 (2006):

arugula
    n 1: erect European annual often grown as a salad crop to be
         harvested when young and tender [syn: rocket, roquette,
         garden rocket, rocket salad, arugula, Eruca sativa,
         Eruca vesicaria sativa]