【Tumor】 , 【neoplasm】 ,  【malignancy】 ,  【cancer】  can all denote an abnormal growth or mass of tissue.

【Tumor】 , the most general term, is applicable to any such growth or mass in or on the surface of the body of a human being, animal, or plant.

【Neoplasm】  often replaces  【tumor】 in technical use when the reference is specifically to a more or less unrestrained new growth of cells that serves no physiological purpose or to a mass formed by such growth.

【Malignancy】  in application to a 【neoplasm】 denotes a growth that because of unrestrained proliferation and tendency to spread and invade other tissues constitutes a danger to life. This use, though deplored by some purists, is common in technical literature and often used euphemistically in discussion with a patient or his associates.

【Cancer】  is the usual popular and technical term for a malignant 【neoplasm】 , though sometimes it is applied specifically to such neoplasms arising in epithelial tissues (as skin or membrane) which are more often distinguished as carcinomas from the other great class of cancers, the sarcomas , that originate in nonepithelial tissues (as bone, muscle, or connective tissue).