The Collaborative International Dictionary of English v.0.48:

Lok \Lok\ (l[o^]k), Loki \Lo"ki\ (l[=o]"k), n. [Icel. Loki,
   perh. akin to lokka, locka to allure, entice.] (Scandinavian
   Myth.)
   The evil deity, the author of all calamities and mischief,
   answering to the Ahriman of the Persians.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Loki
    n 1: (Norse mythology) trickster; god of discord and mischief;
         contrived death of Balder and was overcome by Thor