The area by and around the sea; including the beach, promenade or cliffs
"After a long walk along the promenade, we sat on the seaside to watch the sunset over the cliffs."
In plain English: Seaside is an area near the ocean where you can see waves and swim.
"We went to the seaside for our summer holiday."
Usage: Use "seaside" to refer generally to coastal areas or resorts rather than specific locations like a particular beach. It often implies a vacation setting when used as an adjective before nouns such as "hotel" or "town."
Related to a seaside.
"The hotel offers many facilities related to a seaside, such as private beach access and ocean-view dining."
A city in Monterey County, California, United States.
"The tourists packed their bags to visit Seaside during their summer vacation in Monterey County."
The word seaside comes from Middle English see-side, which is simply the combination of "sea" and "side." It has been used in English since at least the 14th century to describe a location by the ocean.