While the exact legal definition of sexual abuse may vary depending on local laws, the American Psychological Association (APA) defines sexual abuse as, "unwanted sexual activity, with perpetrators using force, making threats, or taking advantage of victims not able to give consent."
While exact laws and wording vary by state in the U.S., in general, individuals that are not able to give consent can be understood to include individuals that are under the age of consent, individuals under the effects of drugs or alcohol, and individuals that are asleep or unconscious.
Many states also have laws stating that individuals under the custody of law enforcement or the care of mental health professionals are not able to consent to sexual activities with said professionals due to the power structure inherent in these situations.
RAINN (Rape, Abuse & Incest National Network) uses the umbrella term "sexual violence" in a similar, non-legal way, to refer to all types of sexual violence, of which it defines sexual abuse as a subset.
Some other categories of sexual violence according to RAINN include sexual assault, intimate partner sexual violence, stalking, and drug-facilitated sexual assault. According to both the APA and RAINN, most victims of sexual abuse and sexual violence know their perpetrator.