Loneliness is a complex and usually unpleasant emotional response to isolation or lack of companionship. It typically includes anxious feelings about the lack of connectedness or communality with other beings, both in the present and extending into the future.

From an evolutionary standpoint, loneliness can be explained as a motivator to stay close to the safety of groups.

Based on information from WikipediaAPA Dictionary of Psychology
All Emotions (56)