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]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "seaside":
   bank, beach, berm, coast, coastal, coastland, coastline,
   embankment, foreshore, ironbound coast, lido, littoral, plage,
   playa, riparial, riparian, riparious, riverside, riviera,
   rockbound coast, sands, sea margin, seabank, seabeach, seaboard,
   seacliff, seacoast, seashore, shingle, shore, shoreline, shoreside,
   shoreward, strand, submerged coast, tidewater, waterfront,
   waterside



U.S. Gazetteer Places (2000):

Seaside, CA -- U.S. city in California
   Population (2000):    31696
   Housing Units (2000): 11005
   Land area (2000):     8.825689 sq. miles (22.858429 sq. km)
   Water area (2000):    0.131820 sq. miles (0.341413 sq. km)
   Total area (2000):    8.957509 sq. miles (23.199842 sq. km)
   FIPS code:            70742
   Located within:       California (CA), FIPS 06
   Location:             36.615810 N, 121.833957 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Seaside, CA
    Seaside


U.S. Gazetteer Places (2000):

Seaside, OR -- U.S. city in Oregon
   Population (2000):    5900
   Housing Units (2000): 4078
   Land area (2000):     3.857824 sq. miles (9.991717 sq. km)
   Water area (2000):    0.156029 sq. miles (0.404114 sq. km)
   Total area (2000):    4.013853 sq. miles (10.395831 sq. km)
   FIPS code:            65950
   Located within:       Oregon (OR), FIPS 41
   Location:             45.992850 N, 123.922108 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Seaside, OR
    Seaside