The Collaborative International Dictionary of English v.0.48:

Seaside \Sea"side`\, n.
   The land bordering on, or adjacent to, the sea; the seashore.
   Also used adjectively.
   [1913 Webster]

WordNet (r) 3.0 (2006):

seaside
    n 1: the shore of a sea or ocean regarded as a resort [syn:
         seaside, seaboard]