Depression can be described as a state of low mood and aversion to activity, which can affect a person's general sense of well-being in a negative manner.    Depressed people often feel sad, anxious, empty, hopeless, worried, helpless, worthless, guilty, irritable, hurt, or restless. They may have problems concentrating, remembering details, feeling emotions, connecting with other people or making decisions. The negative effects of depression may cause the person to contemplate or attempt to commit suicide. Within the context of psychoactive substance usage, depression is often accompanied by other coinciding effects such as anxiety and irritability and can be considered as the polar opposite of cognitive euphoria. It is most commonly induced when a stimulant or depressant is used repeatedly for prolonged periods of time, during the withdrawal symptoms of almost any substance, or during the comedown/crash of a stimulant. However, it is worth noting that substance-induced depression is often much shorter lasting than clinical depression, usually subsiding once the effects or withdrawal symptoms of a drug have ended. When not caused by drug use, in the context of formal psychology, these feelings can be classified as clinical depression or major depressive disorder by the DSM's diagnostic criteria when five (or more) specific symptoms from the list below have been present during the same 2-week period and the feelings represent a change from previous functioning; at least one of the symptoms must also be either (1) depressed mood or (2) loss of interest or pleasure. 
- Depressed mood most of the day, nearly every day, as indicated by either subjective report (e.g., feels sad, empty, hopeless) or observation made by others (e.g., appears tearful). (Note: In children and adolescents, can be irritable mood.)
- Markedly diminished interest or pleasure in all, or almost all, activities most of the day, nearly every day (as indicated by either subjective account or observation.)
- Significant weight loss when not dieting or weight gain (e.g., a change of more than 5% of body weight in a month), or decrease or increase in appetite nearly every day.
- Insomnia or hypersomnia nearly every day.
- Psychomotor agitation or retardation nearly every day (observable by others, not merely subjective feelings of restlessness or being slowed down).
- Fatigue or loss of energy nearly every day.
- Feelings of worthlessness or excessive or inappropriate guilt (which may be delusional) nearly every day (not merely self-reproach or guilt about being sick).
- Diminished ability to think or concentrate, or indecisiveness, nearly every day (either by subjective account or as observed by others).
- Recurrent thoughts of death (not just fear of dying), recurrent suicidal ideation without a specific plan, or a suicide attempt or a specific plan for committing suicide.
- Depression (PsycheCentral) | https://psychcentral.com/disorders/depression/
- Depression (major depressive disorder) | https://www.mayoclinic.org/diseases-conditions/depression/symptoms-causes/syc-20356007
- Depression (National Institute of Mental health) | https://www.nimh.nih.gov/health/topics/depression/index.shtml
- Major Depressive Disorder (DSM Diagnostic Criteria) | https://images.pearsonclinical.com/images/assets/basc-3/basc3resources/DSM5_DiagnosticCriteria_MajorDepressiveDisorder.pdf