Energy exchange is what you look for with tantric sex.
This energy exchange can happen between you and another being.
It can happen as well internally as an energy circulation pattern.
You can for instance focus on energy exchange with the absolute.
This is the universal dimension of your being.
You simply realize that you are united at all times with the life force which surrounds you and you find ways of activating this connection.
You can have a similar type of energy exchange with nature for instance.
You can jump in the fresh waves of the sea and get this intense feeling of union with the elements.
We don’t usually call this sex, right?
It is still some form of energy exchange.
You don’t have to call it tantra either.
This experience did exist long before the term or concept of tantra was created.
So, yes! It is obvious that energy exchange escapes any form of definition.
Tantra is simply a way of calling it.
It is simply to give it a name and a place within a vaster dynamics of things.
You can call this energy exchange whatever you want or whatever you feel comfortable with.
If you don’t have a word or expression coming to mind, you can always use the terms “tantric exchange” or “energy exchange” to describe these experiences.