Goaf, also known as gob or waste, refers to the mined-out area that is left behind after coal has been extracted from a coal seam. Synonyms for goaf include gob pile, spoil heap, coal waste, culm pile, or just plain refuse. These terms all describe the same physical remains of coal mining, but may be more commonly used in certain regions or contexts. Although goaf may seem like an obscure word, it is an important term in the coal mining industry, as understanding the properties and potential hazards associated with goaf is crucial for safe and efficient mining operations.