Here's is what I understand so far and the following is referenced directly from UNSW CHEM1061 lecture notes by Professor Pall Thordarson:
order of the reaction SHOULD be equal to the number of molecules of that reactant involved in each collision ( however this is not always the case)
it is only possible to predict the order of a reaction is it occurs in a single step (elementary reactions)
reaction order is experimentally derived from rate equation
so I understand all of the above but what I fail to fully comprehend is that actual DEFINITION of 'reaction order' what is it in it's entirety, what does it's value represent if it only sometimes correlates with molecularity (no. of molecules which react in an elementary reaction) ? If someone can answer this question could they please provide a qualitative answer that explains the value conceptually? Or is it just a value to relate concentration to reaction rate? that helps us work out more information?