(1) combined processes (such as sublimation, fusion or melting, evaporation) which remove snow or ice from the surface of a glacier or from a snow-field; also used to express the quantity lost by these processes (2) reduction of the water equivalent of a snow cover by melting, evaporation, wind and avalanches

National Snow & Ice Data Center Cryosphere glossary https://nsidc.org/cryosphere/glossary

Ablation is the natural removal of snow or ice from the surface of a glacier or snowfield. This can occur through melting or sublimation and, at or near the snout of a glacier, it can also occur by calving. The area of a glacier where the removal of snow and ice is greater than the amount replaced each year is called the Ablation Zone or Ablation Zone Region. In other words, ablation can occur at any point on the surface of a glacier, but the ablation zone of a glacier is the part where there is a net loss of surface snow and ice

http://www.geography-site.co.uk/pages/physical/glaciers/ablation.html

https://www.nap.edu/read/13449/chapter/4#23