Depression
Short definition
Depression is a mental disorder characterised by depressed mood or loss of interest and pleasure in activities, for most of the day, nearly every day, for at least two weeks.
In plain language
It goes beyond passing sadness: it can bring changes in sleep and appetite, tiredness, difficulty concentrating, feelings of guilt and, in severe cases, thoughts of death. In older people it may present less typically and be mistaken for cognitive decline. It is treated with psychotherapy, medication or both, depending on severity.