BDSM is primarily about acceptance, and your willingness to accept things that you don't normally accept, in a sexual context.
In this diagram, we see three zones:
BDSM is about exploring the yellow/orange zone.
Sometimes there are things we accept, but only grudgingly. In BDSM, this doesn't count. Acceptance must be complete.
So we can re-label the Venn diagram more precisely thus:
A BDSM session is an activity where two people with some kind of sexual connection act in a manner such that at least one of them explores his or her willingness to completely accept certain things which they do not normally accept.
The most important feature of a BDSM session is its boundary.
Here is an example of a session with a boundary added to the Diagram of Complete Acceptance:
In this case, the boundary lies entirely within the yellow/orange and green zones.
Within the green zone, the session is relatively boring (although anticipation may be felt); in the yellow/orange zone it becomes interesting.
If both parties in the session clearly understand that the session boundary is one which does not extend into the red zone, then one or both parties can achieve complete acceptance more easily, simply by deciding to "take whatever comes".
However, it can be difficult to define session boundaries which completely avoid the red zone, yet which allow the possibility of visiting all interesting locations within the yellow/orange zone. So in practice a pre-defined session boundary may not completely exclude the red zone.
For such a session, some kind of "safe word" will be required.
Here is a session with a boundary that extends a little bit into the red zone:
(Note: there is no single official BDSM convention about safe words, but sometimes color words like "red", "orange", "yellow" and "green" are used with meanings corresponding to my useage of those colors in the diagrams in this article.)