Homophobia encompasses a range of negative attitudes and feelings toward homosexuality or people who are identified or perceived as being lesbian, gay or bisexual. It has been defined as contempt, prejudice, aversion, hatred or antipathy, may be based on irrational fear and may also be related to religious beliefs. Negative attitudes towards transgender and transsexual people are known as transphobia.

Homophobia is observable in critical and hostile behavior such as discrimination and violence on the basis of sexual orientations that are non-heterosexual. Recognized types of homophobia include institutionalized homophobia, e.g. religious homophobia and state-sponsored homophobia, and internalized homophobia, experienced by people who have same-sex attractions, regardless of how they identify.

Negative attitudes toward identifiable LGBT groups have similar yet specific names: lesbophobia is the intersection of homophobia and sexism directed against lesbians, gayphobia is the dislike or hatred of gay men, biphobia targets bisexuality and bisexual people, and transphobia targets transgender and transsexual people and gender variance or gender role nonconformity. According to 2010 Hate Crimes Statistics released by the FBI National Press Office, 19.3 percent of hate crimes across the United States "were motivated by a sexual orientation bias." Moreover, in a Southern Poverty Law Center 2010 Intelligence Report extrapolating data from fourteen years (1995–2008), which had complete data available at the time, of the FBI's national hate crime statistics found that LGBT people were "far more likely than any other minority group in the United States to be victimized by violent hate crime."