In Law negotiorum gestio is a form of agency wherein a gestor acts on behalf and for the benefit of a principal without the latter's prior consent.
For example, while you are traveling abroad, a typhoon hits your home town and the roofing of your house is in danger. To avoid the catastrophic situation, your neighbour does something urgently necessary. You are the 'principal' and your neighbour here is the 'gestor', the act of which saved your house is the negotiorum gestio.
It seems that it is much more important that the business (the matter) does not pertain to the gestor (to which matter he has no mandate or any legal responsibility otherwise to act either).