Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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