What is authentic food?

I often go to restaurants and people always tell me that it's not authentic.

I dont understand how a certain food is not authentic..different parts of china/mexico/vietnam/ lebanon etc have different type of ways to cook "authentic foods".

I recently went to a pho restaurant and someone told its not authentic because it doesnt taste the same as the pho his parent's cook..and they are born and from vietnam.

I just told him to shove it..authentic to him or not..imma eat it LoL.

What's your take on authentic food?