The law definition in the United States, you mean.
In Poland, it requires use of force or coercion on part of the person doing the sexual act.
Without it, say drugging and then letting someone else do it, it's "just" assault. (The other person is committing sexual assault.)
What Epstein did is called pimping and also punishable by law.
Potentially also abduction or assault.
Forcing a minor to commence a sexual act is a separate category of pimping (pimping a minor) that's yet more harshly punished.
Sex with minor is also punishable by law separately. (With some exceptions and discretion by the judge.) Now there is a strong association with sexual assault, though circumstances can leave it as just sex with minor. Depends on prosecutor and judge.
So it is likely Epstein would be convicted of multiple crimes and potentially recidivism (repeat offense) and organized crime, while Minsky would be convicted for sex with minor, not any kind of assault.
Note that repeat offense means Epstein would not be offered bail nor allowed a reduced sentence.