This is a digit that can be placed opposite the fingers of the same hand and therefore allows the hand to grasp and handle objects, characteristic of primates.

Answer;

Opposable thumb

Explanation;An opposable thumb is a thumb that can be placed opposite the fingers of the same hand. Opposable thumbs allow the digits to grasp and handle objects and are characteristic of primates.Opposable thumbs are one of the distinguishing features of primates. Opposable  thumbs of human beings allow them to use tools.