Ablation
The amount of the glacier that is lost through melting and evaporation, normally on lower ground
Calving
When icebergs fall off the glacier at its snout
Advance
The growth of a glacier when accumulation exceeds ablation
Retreat
The shrinking of a glacier uphill when ablation exceeds accumulation
Accumulation
The amount of new snow that is gained by a glacier each year, normally on high land.