Loneliness is an unpleasant emotional response to perceived isolation. Loneliness is also described as social pain – a psychological mechanism which motivates individuals to seek social connections. It is often associated with a perceived lack of connection and intimacy. Loneliness overlaps and yet is distinct from solitude. Solitude is simply the state of being apart from others; not everyone who experiences solitude feels lonely. As a subjective emotion, loneliness can be felt even when a person is surrounded by other people. Hence, there is a distinction between being alone and feeling lonely. Loneliness can be short term (state loneliness) or long term (chronic loneliness). In either case, it can be intense and painful.

The causes of loneliness are varied. Loneliness can be a result of genetic inheritance, cultural factors, a lack of meaningful relationships, a significant loss, an excessive reliance on passive technologies (notably the Internet in the 21st century), or a self-perpetuating mindset. Research has shown that loneliness is found throughout society, including among people in marriages along with other strong relationships, and those with successful careers. Most people experience loneliness at some points in their lives, and some feel it often. The effects of loneliness are also varied. Transient loneliness (loneliness which exists for a short period of time) is related to positive effects, including an increased focus on the strength of one's relationships. Chronic loneliness (loneliness which exists for a significant amount of time in one's life) is generally correlated with negative effects, including increased obesity, substance use disorder, risk of depression, cardiovascular disease, risk of high blood pressure, and high cholesterol. Chronic loneliness is also correlated with an increased risk of death and suicidal thoughts.

Medical treatments for loneliness include beginning therapy and taking antidepressants. Social treatments for loneliness generally include an increase in interaction with others, such as group activities (such as exercise or religious activities), re-engaging with old friends or colleagues, and becoming more connected with one's community. Other social treatments for loneliness include the ownership of pets and loneliness-designed technologies, such as meetup services or social robots (although the use of some technologies in order to combat loneliness is debated).

Loneliness has long been a theme in literature, going back to the Epic of Gilgamesh. However, academic coverage of loneliness was sparse until recent decades. In the 21st century, some academics and professionals have claimed that loneliness has become an epidemic, including Vivek Murthy, the Surgeon General of the United States. However, this claim has been disputed, with critics arguing that loneliness has not increased, but rather only academic focus on the topic has.