what is mother tongue ? do you know exact definition?

recently i had a debate with one of my colleague about mother tongue though for fun i said my mother tongue is "Kannada" which is a regional language of the state where i live and settled here from 1930's almost 80 years in Bangalore even my father and me was born and bought up in Bangalore but in home i speak Telugu, but he insisted that my mother tongue is Telugu he was saying it from the perspective of my ancestral roots and origin, but i was talking in the angle of region and language which i am most used from my birth so this made me think what really mother tongue is? a "fill up the blank" that i am filling from my childhood in all the applications from my studies to job but yet still never tried to find whats the exact meaning it!

After searching on net for a while i came to know that MT has no predefined meaning and terminology is used in different way in different countries in some countries mother's language is considered as mother tongue, in some countries language most used to or native language is considered as mother tongue, in some part of africa respective tribal language is considered as mother language even though all of them reside in same region and in india ancestral origin or roots of family language(ethnicity) is considered as mother tongue even though he don't know to speak the language for example my friends mother tongue is konkani but he don't know to speak or even understand the language weird isn't it.

Have your ever thought of what MT is? or just was filling up the blank like me? so do you agree with the Indian definition of mother tongue or shud it be changed and between what if your mothers mother tongue is different and fathers mother tongue is different what would you choose then :) just penn in u r comments.