(CNN)Controversial comments made by "Harry Potter" author J.K. Rowling have sparked another online discussion about gender identity.
Rowling came under fire over the weekend for a tweet in which she mocked a headline that read, "people who menstruate."
"'People who menstruate.' I'm sure there used to be a word for those people," she tweeted. "Someone help me out. Wumben? Wimpund? Woomud?"
Amid the backlash in which Twitter users accused her of transphobia, Rowling doubled down on her initial comment, saying that "erasing the concept of sex removes the ability of many to meaningfully discuss their lives. It isn't hate to speak the truth."
The phrase "people who menstruate" was intended to be inclusive of women, non-binary people, trans men and gender-fluid and gender-nonconforming people.
Rowling said that if "sex isn't real," the realities of cisgender women (women whose gender identity conforms with the sex they were assigned at birth) globally are erased.
But the people she excluded in her initial tweet were trans men, non-binary and gender-fluid people, by emphasizing the idea that it's only cisgender women who menstruate. It also left out trans women by implying that if they cannot menstruate, they are not women.
Trans women are people who were assigned male at birth but identify as women. Trans men are people who were assigned female at birth but identify as men.
Her comments were a "fundamental misconception of the difference between sex and gender," said Amit Paley, CEO and executive director of The Trevor Project, a suicide prevention and crisis intervention organization for LGBTQ youth.
"Your sex is what you are assigned at birth, and your gender is the way that you experience your own sense of gender identity," Paley said. "It's important [that we recognize] very clearly that trans women are women and that trans men are men. ... and that we respect people for the way that they identify and live their own gender."
Rowling's tweets also "demonstrate a clear misunderstanding of who trans people are, what we want, what we need and what the medical consensus is," said Sarah McBride, a transwoman and national press secretary of the Human Rights Campaign, a civil rights organization working to achieve equality for LGBTQ Americans.
"Beyond that," she added, Rowling's tweets "are part of a prejudice that results in discrimination and violence in too many circumstances."
A representative for Rowling did not immediately respond to CNN's previous request for comment regarding the situation.
Understanding the difference between gender and sex is important to understanding the spectrum of experiences based on the two concepts. Here's what gender and sex mean and how they impact a person's identity.
Defining gender vs. sex
Some medical associations have recognized the difference between gender and sex.
The American Medical Association, the American Psychological Association and the American Psychiatric Association, as well as centuries of transgender people's lived experiences, have affirmed transgender people, McBride said.
Gender refers to the attitudes, feelings and behaviors that a culture associates with a person's biological sex, according to the American Psychological Association. In other words, gender is a social construct and a social identity.