The term 'Hollow Organ' can be summarized by the crossword solution 'CYST', which is a four-letter word. A CYST is a closed sac or capsule that is filled with fluid, gas, or semi-solid material. It is typically found in the body and can be caused by infection, clogging of a gland or duct, or a tumor. Cysts can be benign or malignant and can occur in any organ or tissue. Treatment for cysts depends on the type and severity of the CYST.