noun
common gull; mew gull
Specifically refers to the common gull (Larus canus). In everyday Japanese, カモメ often covers seagulls in general, but this sense is the precise ornithological meaning.
カモメ（Larus canus）は中型のカモメ科の鳥です。
The common gull (Larus canus) is a medium-sized gull.
noun
seagull; gull
General term for seagulls and gulls (family Laridae). This is the most common everyday meaning.
海岸でカモメが鳴いている。
Seagulls are crying at the beach.
カモメが波の上を飛んでいる。
A seagull is flying over the waves.
Kanji form; rarely used in modern writing. It is a rarely used kanji form.
ウミネコ (black-tailed gull) is a specific species often seen in Japan, while カモメ is a broader term for gulls in general.
The origin is uncertain. The word is native Japanese, and the kanji forms 鴎 and 鷗 are Chinese characters adopted for their meaning.