Sylvia Plath's The Bell Jar is a thinly-veiled autobiography of her juvenilia.  From the beginning, Plath characterizes her narrator Esther with lyrical imagery related to sickness and death:

 "I'm stupid about executions..."

"The idea of being electrocuted makes me sick..." 

"...pretty soon I felt as though I were carrying that cadaver's head around with me on a string, like some black, noseless balloon stinking of vinegar"

"I feel gawky and morbid as somebody in a sideshow"

 "[I am]...the negative of a person."

A bit later, Esther and girls will become ill at the luncheon in New York, which symbolizes the sickness of the paternal culture for women in the 1950s.  Plath's imagery here foreshadows Esther's mental illness later in the novel. Later, Ether will feel trapped by the culture like a specimen in a bell jar.  

Plath also characterizes Esther using many foils and alter-egos.  Foils in The Bell Jar serve to stifle Esther's decision-making by presenting two competing lifestyles which she cannot reconcile, thus compounding her flaws and making her more sick.  As in her poetry, Plath uses mirror motifs in her characterization, paralleling Esther and Esther's alter ego, Elly Higginbottom, and pitting her feminist heroines against their paternal reflections.   To Esther, these two gender roles are mutually exclusive, for males in the pre-sexual revolution have the freedom to choose from both sides of the fig tree, but women must choose one or the other.  Esther wants to choose feminism, but she's been raised to choose the paternal path, so she chooses neither, retreating instead toward isolation and bitterness toward materialism, conformity, and the sexist social structure.

The Bell Jar is replete with foils and alter egos.  Plath herself first published the novel under the pseudonym Victoria Lucas.  Then there's her alias, Esther Greenwood, and her alias, Elly Higginbottom, a worldly girl who downs Vodka straight.  Esther even envisions her future pen name as "Ee Gee," a copy of her mentor Jay Cee.  As in her poetry, Plath's distinctions between author and persona are playful reflections of one another, as if the mirror image, or persona, is how Plath wants society to see her. For every side of Esther, there is a foil to reflect it, as if she wants the reader first to examine the shards of her broken identity and then take a step back to observe the pointillism portrait of an emerging feminist who cannot quite be put back together.

One critic has said that reading The Bell Jar is like ride at funhouse with dizzying mirrors that make you both sick and exhilarated.  To be sure, Plath's use of lyricism, morbid imagery, foils, and alter-egos serve to distort and sicken her characters and the reader. 

In The Bell Jar, what terms does Esther Greenwood use to describe herself?

One of the most important aspects of this novel is the way that it represents a very introspective self-examination of a young woman who reveals herself under the microscope, as it were, in her narrative. As a result, the novel contains many compelling descriptions of Esther. You might like to consider the following example from Chapter Seven:

When I was nineteen, pureness was the great issue. Instead of the world being divided up into Catholics and Protestants or Republicans and Democrats or white men and black men or even men and women, I saw the world divided into people who had slept with somebody and people who hadn’t, and this seemed the only really significant difference between one person and another. I thought a spectacular change would come over me the day I crossed the boundary line.

This quote is significant because it presents Esther as occupying a universe where her sexual opportunities are extremely curtailed. Tradition necessitates that she has no sexual exploits before marriage, as breaking this taboo threatens her future in a somewhat Puritanical society due to the possible dangers of crossing that line. Her goal to deliberately transgress this social norm is met, though with somewhat indifferent results. When she finally manages to succeed in her ambition of losing her virginity, it is a mixed experience, as she fails to experience the sense of liberation that she hopes for.

This quote therefore shows Esther to be a young woman who is at odds with the world of which she is a part and how she desires to break social norms and conventions. This is of course just one quote, so I hope you are able to find and analyse others in this narrative.