hi Patrizia
Your question is far from stupid, however it may have two sides to it:
Do you mean what defines something as LEGO SERIOUS PLAY – in that case we would often turn to the core process. If one does not adhere to the core process, then it is not LSP
Do you mean defining what it does or is, ie a method etc – in this case we often talk about it either as a langauge; or problem solving and dialogue method that unlocks the full potential. In our forthcoming book Robert and I talk about as a method that helps groups go beyond 80/20 meeting, helps break habitual thinking and helps to lead to unlock (potential and potential)
As to do we need a definition, I do believe that agreen in what defines it is essential, and I venture to say building our time at LEGO we (The Association of Master Trainers) have settled on the core process. For defining what it does or is, this is probably also important, but may be more fluid, partly due to langauge differences, partly because as we all get more experiences about what the core process can do then we get a better and broader definition.
This was as far as I could stretch my brain on a Saturday morning
Play well ;-)