In physical terms, to turn a hollow structure into itself – for example, a length of the intestine may ‘enter’ the succeeding portion, also known as INTUSSUSCEPTION.

A psychological term to describe what happens when an individual is more interested in his or her ‘inner world’ than in what is happening around in the real world. An INTROVERT tends to have few friends and prefers to persist in activities that they have started. Karl Jung (see JUNGIAN ANALYSIS) described introversion as a person's tendency to distance him or herself from others; to have philosophical interests; and to have reactions that are reserved and defensive.