A measure of the change in mass of a glacier at a certain point for a specific period of time. The balance between accumulation and ablation. Also called Mass Budget. The change in the mass of a glacier, or part of the glacier, over a stated span of time; the term mass budget is a synonym. See mass-balance units for recommended units. The span of time is often a year or a season. A seasonal mass balance is nearly always either a winter balance or a summer balance, although other kinds of season are appropriate in some climates, such as those of the tropics. The definition of year depends on the method adopted for measurement of the balance. See time system. The reference in the definition to a glacier means that a particular volume of space is being studied. A properly delineated glacier has no mass transfer of ice across its boundary other than as ice discharge. However, the mass balance is often quoted for volumes other than that of the whole glacier, for example a column through the glacier, the part of the glacier upglacier or downglacier from the grounding line, or a band defined by two contours of surface elevation. It is necessary in such cases to make clear that the study volume is something other than the whole glacier, and also to make clear which components of the mass balance are being reported. The quantity reported may be the climatic mass balance or the climatic-basal mass balance, but will often be the surface mass balance. In all cases the need for a defined study volume is fundamental because without it the principle of conservation of mass cannot be invoked. The study volume may change over the study period. The surface and bed elevations may change, and the areal extent is unlikely to be the same at the end of the period as it was at the beginning. Whether these changes are significant will depend not just on their magnitude and the accuracy with which they can be determined but on the purpose of the investigation. See conventional balance, reference-surface balance.