NCI: In medicine, a lump in the body. It may be caused by the abnormal growth of cells, a cyst, hormonal changes, or an immune reaction. A mass may be benign (not cancer) or malignant (cancer).,NCI: A benign or malignant pathologic structure in any part of the body resulting from cystic changes or accumulation of inflammatory or neoplastic cells.,NCI: A benign or malignant pathologic structure in any part of the body, resulting from a neoplastic accumulation of cells, inflammatory cells, or cystic changes.