When vaginal penetration is about to occur, but hasn't happened yet. This includes sex, sex toys, tentacles, fingering, or any other type of penetration.
Don't use this tag if you don't see penetration about to occur. This differs from grinding in that grinding is about rubbing the penis against the vagina, while imminent vaginal is about the penis being about to penetrate the vagina.
If the penetration is non-consensual, tag imminent rape.
Also, try not to confuse this with after vaginal, which may look similar to this in small thumbnails.
This tag implicates imminent_penetration (learn more).