The Collaborative International Dictionary of English v.0.48:

Seasickness \Sea"sick`ness\, n.
   The peculiar sickness, characterized by nausea and
   prostration, which is caused by the pitching or rolling of a
   vessel.
   [1913 Webster]

WordNet (r) 3.0 (2006):

seasickness
    n 1: motion sickness experienced while traveling on water [syn:
         seasickness, mal de mer, naupathia]