A caloric deficit is any shortage in the amount of calories consumed relative to the amount of calories required for maintenance of current body weight (energy homeostasis).
A deficit can be created by reducing input/calories consumed (lower food intake, aka dieting). A deficit can also be created by increasing output without a corresponding increase in input. Increased output is created by increasing physical activity, from increased caloric requirements necessary to heal an injury, or from growth. There are also some substances, including Caffeine, which can create a small (3-5%) increase in caloric expenditure, via a variety of pathways that include increasing physical activity levels and increasing Thermogenesis (heat output), and/or by reducing caloric input via appetite suppression. Drugs and herbal treatments creating a more extreme metabolic effect exist; however, they cause extreme increases of heart rate and thermogenesis that can cause death in even very healthy and athletic individuals, and these drugs are not widely sold.
As the calories required for energy homeostasis decreases as the organisms's mass decreases, if a moderate deficit is maintained eventually a new (lower) weight will be reached and maintained, and the organism will no longer be at caloric deficit. A permanent severe deficit, on the other hand, which contains too few calories to maintain a healthy weight level, will eventually result in starvation and death.