Enthusiastic consent: Saying “yes” to any sexual act, and meaning it.
A definition from Feministe:
The basic idea is simple: don’t initiate sex unless you have your partner’s enthusiastic consent. Not a partner who says, “Okay, I guess,” in a bored tone, but doesn’t actively say “no”. Not a partner who is silent and non-reactive, but doesn’t actively stop you when you start having sex with them. Not a partner who seems hesitant, or anxious, or confused. Enthusiastic consent means an enthusiastic partner: one who is responding passionately, kissing you back, saying things like “Yes” or “Oh my God, don’t stop!”