What exactly defines consent to sexual intercourse?

"What exactly defines consent to sexual intercourse?"

It isn't universally defined.  In other words, the law varies from state to state, to country to country.  There is no universal law, because since the beginning of history, this is one of those primal human dances that cannot be quantified, as it is so varied and functionally dynamic, depending on so many base factors, only beginning with the two diverse individuals involved.

So the gold-standard that gynocentric feminists demand is that women can remove consent at any point at all.  But male physiology doesn't work that way.  If there were some men's groups that would speak on this issue, they might say that consent is given by a woman once she agrees to begin the romantic involvement, such as kissing in a private space, since men can't "unconsent" past a certain point.  The reality is that for any given encounter, things are all different.  And consent is a subjectively driven concept dependent on so many parameters.

I'm afraid that this consent to sex idea will remain an elusive endeavour to nail down into law, much like the abortion/right to life debate.  These things just aren't that simple.

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To consent is to give permission for something to happen.

As someone who worked in higher education for many years and have heard a great deal about college sexual assault cases, the issue however is rarely about whether or not consent was given, the issue is whether or not the consent was valid.

A woman giving consent after drinking is one reason consent has long been judged to be invalid if the woman later regrets having sex.  Oddly this never seems to apply to men however.   In more recent years another reason consent has often been ruled invalid is the idea that many women feel powerless to say no to men and feel they can only say yes.  Therefore it's ruled that while a woman said yes, she was thinking no so therefore the consent was invalid, meaning consensual consent was rape. This of course is very problematic, but that's the trend.  

Verbal encouragement while and before you are doing it...  or him/her putting the condom around your pecker...