The "bell jar" is a symbol that appears throughout the novel The Bell Jar by Sylvia Plath, and it is a metaphor that Esther uses to explain how she feels suffocated by the world. The bell jar is used in laboratories to create vacuums and, in Plath's novel, symbolizes society and the suffocating limitations it imposes; the difficulties of life, the uncertainties of life. There are many quotes that we can analyze where she mentions the bell jar.
"...wherever I sat -- on the deck of a ship or at a street café in Paris or Bangkok -- I would be sitting under the same glass bell jar, stewing in my own sour air."
The bell jar is a suffocating container that Esther feels has been placed over her, and many other people, is the restraints that society places on individuals, expectations, and the difficulties of life. Esther often feels trapped and doesn't know what she wants or how to get out. She also says,
"To the person in the bell jar, blank and stopped as a dead baby, the world itself is the bad dream."
In this quote, she explains that the person trapped in the bell jar, trapped by whatever it is in life that makes a person feel that way, is caught in a bad dream.
But the bell jar is not always there, enveloping Esther. At one point she says, "The bell jar hung, suspended, a few feet above my head. I was open to the circulating air." She is starting to feel better, like she can breathe again, like maybe the restraints of life are starting to fade a little.
At the end of the novel, even though Esther is doing much better and is out of the mental institution, and at that moment doesn't feel trapped, but worries that the bell jar, the difficulties, will return. She says,
"How did I know that someday -- at college, in Europe, somewhere, anywhere -- the bell jar, with its stifling distortions, wouldn't descend again?"
She feels she has not escaped the bell jar (the pressures of society) for good, but at least she has for the moment.