Feral children (aka wild children) are children who have been isolated from human contact at a very early age. They do not experience the human love, care, intimacy, socialization, and language need to develop and mature healthily. Some are said to be raised by animals and some lived in the wild on their own.
Contrary to fictional examples, like Tarzan, feral children do not experience superior strength, survival instincts, and normal human intelligence. In fact, most have minimal social skills and have missed a crucial period in their life to learn language. When discovered much of the damage is irreversible.
To learn more you can watch the documentary Wild Child: The Story of Feral Children