What is the difference between mother tongue and second language ?

Mother tongue refers to the first language that a person learns and speaks from birth or early childhood. It is the language that is spoken at home and is typically the language of the person's parents or caregivers. Second language, on the other hand, refers to any language that a person learns in addition to their mother tongue. This could be a language learned in school, through travel, or through other means. The key difference is that mother tongue is the primary and most fluent language of a person, while a second language is learned later in life.