Depression is a mood disorder and is defined as a lowering of the mood towards states of sadness and melancholy.
People suffering from depression feel sad, useless, failed, have no interest in sociability or in activities that previously interested them.
A peculiarity are ruminative thoughts and a much more present counterfactual thought: people suffering from depression tend to rethink actions performed in the past and to think about how it would have gone if.
Depression is often associated with sleep disorders: insomnia or hypersomnia and also with eating disorders (eating less or more -> nervous hunger, if you do not pay attention to this aspect you can also incur eating disorders such as bulimia and anorexia).
A common mistake is to use the word “depression” to designate a momentary state of sadness. Everyone happens to be sad on certain days or at certain times of their life and it is important that it is so: sadness is important as it allows us to better appreciate happiness and to reflect on our actions.
Depression, on the other hand, is a disease that requires psychotherapeutic interventions as it has effects that prevent the person from living effectively and happily, from entering the social context and facing everyday life.