One of those common words that is hard to define.
A seagull is a gull that lives near the sea.
A gull is a member of the family Laridae, subfamily Larinae. But not all members of Larinae are gulls. Most gulls are part of the Genus Larus. Some are not. It's not a very technical term.
So.... A seagull is a stout, web-footed, mostly-probably white or grey with black markings, omnivorous bird of the subfamily Larinae that you see when you're at the beach or out on a boat.
More exactly, a seagull is a loud, fat bird that hangs out around any group of humans that comes near the sea, hoping to pick up some food-like scraps to eat. They often travel in packs, and are usually primarily white. Anything else is overanalysis.