The following definition comes directly from the US Department of Education Office for Civil Rights.
Title IX is a federal civil rights law passed as part of the Education Amendments of 1972. Title IX requires schools to take steps to prevent and remedy two forms of sex-based harassment: sexual harassment (including sexual violence) and gender-based harassment. Sexual harassment is unwelcome conduct of a sexual nature. It includes unwelcome sexual advances, requests for sexual favors, and other verbal, nonverbal, or physical conduct of a sexual nature. Sexual violence is a form of sexual harassment. Sexual violence, as OCR uses the term, refers to physical sexual acts perpetrated against a person’s will or where a person is incapable of giving consent. A number of different acts fall into the category of sexual violence, including rape, sexual assault, sexual battery, sexual abuse, and sexual coercion.
Title IX also prohibits gender-based harassment, which is unwelcome conduct based on a student’s sex, harassing conduct based on a student’s failure to conform to sex stereotypes.
Sex-based harassment can be carried out by school employees, other students, and third parties. All students can experience sex-based harassment, including male and female students, LGBT students, students with disabilities, and students of different races, national origins, and ages. Title IX protects all students from sex-based harassment, regardless of the sex of the parties, including when they are members of the same sex.
Sex-based harassment creates a hostile environment if the conduct is sufficiently serious that it denies or limits a student’s ability to participate in or benefit from the school’s program. When a school knows or reasonably should know of possible sex-based harassment, it must take immediate and appropriate steps to investigate or otherwise determine what occurred. If an investigation reveals that the harassment created a hostile environment, the school must take prompt and effective steps reasonably calculated to end the harassment, eliminate the hostile environment, prevent its recurrence, and, as appropriate, remedy its effects.