gull
Synonym discussion
Sense: noun/1
"Gull" is the standard general name for this bird family, while "seagull" is the everyday popular word many people use for the same birds, especially near beaches. In careful writing, "gull" is often better because some species live inland as well as on the coast. "Tern" names a different but related seabird that is slimmer, lighter, and usually more sharply pointed in shape and flight. "Seabird" is the broadest term of all: it can refer to gulls, terns, puffins, and many other birds that spend much of their lives near the sea. If you mean the familiar grey-and-white bird stealing food on the beach, both "gull" and "seagull" are natural choices.
Etymology
The bird name "gull" is very old in the languages spoken around the British coast. English is thought to have taken it from a Celtic source related to Welsh gwylan and Breton gwelan, both meaning a gull. Later English also developed figurative uses for a person who is easy to cheat and for the act of cheating someone. Those later meanings are much newer than the original bird sense.