Symbolism, Imagery, Allegory
The lake has a hypnotizing power over the characters in The Seagull. Dorn credits it with making everyone fall (unhappily) in love:
Dorn: You're so upset! You're all so upset! All this love… It's that magic lake! (Tenderly [To Masha]) But what can I do to help, my child? Hm? What? What? (1.188)
Nina confesses that she's drawn to the lake just like a seagull is. For Konstantin, it's a beautiful, natural setting for his experimental play, far superior to a box set in some stuffy theater. The lake makes Trigorin want to fish quietly all day long… and write stories about young women who live by it:
Trigorin: Idea for a short story. The shore of a lake, and a young girl who's spent her whole life beside it… she's happy and free as a seagull. Then a man comes along, sees her, and ruins her life because he has nothing better to do. Destroys her like this seagull here. (2.117)
While the lake means different things to each character, we'd say it symbolizes the longing of all of them—the wish they have for balance, peace, and happiness. And that wish is never going to come true, no matter how hard they wish upon stars (or seagulls) for it to happen.