The common name for a malignant neoplasm or tumor. Neoplasms are new growths and can be divided into benign and malignant types, although in some instances the distinction is unclear. The most important differentiating feature is that a malignant tumor will invade surrounding structures and metastasize (spread) to distant sites whereas a benign tumor will not. Other distinctions between benign and malignant growth include the following: malignancies but not benign types are composed of highly atypical cells; malignancies tend to show more rapid growth than benign neoplasms, and are composed, in part, of cells showing frequent mitotic activity; and malignant tumors tend to grow progressively without self-limitation.