California has become the first US state to require students on state-funded campuses to have clear, active consent before all sexual activity.
Governor Jerry Brown signed the "yes means yes" bill, which advocates say will change the perception of rape.
The legislation stipulates that voluntary agreement, rather than lack of resistance, defines consent.
In January President Obama launched an initiative to combat sexual assault, particularly on college campuses.
"Yes means yes" is the first law in a US state to make the language of affirmative consent a central principle of school sexual assault policies.
The rule defines consent as "an affirmative, conscious and voluntary agreement to engage in sexual activity."
Lawmakers say, however, that consent can be non-verbal, if it is unambiguous.
The aim is to improve the way that campuses deal with accusations of sexual assault, and to challenge the notion that victims of sexual crimes need to have resisted assault in order to have valid complaints.
The legislation also says that silence or a lack of resistance do not constitute consent. Under the bill, someone who is drunk, drugged, unconscious or asleep cannot grant consent.
Sofie Karasek, an activist from the University of California-Berkeley told San Jose Mercury News that she believed the bill would change the cultural perception of rape.
"There's this pervasive idea that if it's not super violent then it doesn't really count," she said.
However critics say that the new law dangerously expands the definition of assault.
The National Coalition For Men condemned the bill as "misandric" (man-hating), saying it "gives license to false accusations, denies the accused due process, will cost the state hundreds of millions of dollars in law suits and sets the stage for ruining innocent lives."
The Department of Education has named dozens of colleges and universities under investigation for mishandling sexual assault cases.
The US estimates one in five women is sexually assaulted while at university.