Loneliness is a feeling of isolation in your own experience. Most commonly, people think about loneliness as a result of physically being alone or without companions. While that is the most common and literal form of loneliness, it can extend far beyond not being surrounded by people. People can also feel lonely when they believe that nobody understands their experience. Frequently loneliness is directly connected to lack of social support. Studies have shown that worldwide feelings of loneliness have doubled in the past 30 years.
FOMO (or fear of missing out) is a slightly different phenomenon that refers to when an individual has a feeling that other people have a positive experience without that individual. This leads to feelings of loneliness as the person is now isolated from the experience that everybody else is having. Loneliness and FOMO are natural feelings that everybody has at one point or another and do not constitute depression on their own. But when those emotions become pervasive and a part of every day, they can lead to depression.