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tissue composed of cells that grow in an abnormal way. Normal tissue is growth-limited, i.e., cell reproduction is equal to cell death. Feedback controls limit cell division after a certain number of cells have developed, allowing for tissue repair but not expansion. Tumor cells are less responsive to these restraints and can proliferate to the point where they disrupt tissue architecture, distort the flow of nutrients, and otherwise do damage.

Tumors may be benign or malignant. Benign tumors remain localized as a discrete mass. They may differ appreciably from normal tissue in structure and excessive growth of cells, but are rarely fatal. However, even benign tumors may grow large enough to interfere with normal function. Some benign uterine tumors, which can weigh as much as 50 lb (22.7 kg), displace adjacent organs, causing digestive and reproductive disorders. Benign tumors are usually treated by complete surgical removal. Cells of malignant tumors, i.e., cancerscancer,
in medicine, common term for neoplasms, or tumors, that are malignant. Like benign tumors, malignant tumors do not respond to body mechanisms that limit cell growth.
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, have characteristics that differ from normal cells in other ways beside cell proliferation. For example, they may be deficient in some specialized functions of the tissues where they originate. Malignant cells are invasive, i.e., they infiltrate surrounding normal tissue; later, malignant cells metastasize, i.e., spread via blood and the lymph system to other sites.

Both benign and malignant tumors are classified according to the type of tissue in which they are found. For example, fibromas are neoplasms of fibrous connective tissue, and melanomas are abnormal growths of pigment (melanin) cells. Malignant tumors originating from epithelial tissue, e.g., in skin, bronchi, and stomach, are termed carcinomas. Malignancies of epithelial glandular tissue such as are found in the breast, prostate, and colon, are known as adenocarcinomas. Malignant growths of connective tissue, e.g., muscle, cartilage, lymph tissue, and bone, are called sarcomas. Lymphomas and leukemiasleukemia
, cancerous disorder of the blood-forming tissues (bone marrow, lymphatics, liver, spleen) characterized by excessive production of immature or mature leukocytes (white blood cells; see blood) and consequently a crowding-out of red blood cells and platelets.
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 are malignancies arising among the white blood cells. A system has been devised to classify malignant tissue according to the degree of malignancy, from grade 1, barely malignant, to grade 4, highly malignant. In practice it is not always possible to determine the degree of malignancy, and it may be difficult even to determine whether particular tumor tissue is benign or malignant.

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An aberrant new growth of abnormal cells or tissues; a tumor.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
The latter, however, usually occurs in a somewhat younger age group, although there is overlap, and characteristically has a more worrisome gross appearance. On microscopic examination there is greater uniformity to the cystic pattern in the microcystic stromal tumor than in the reticular-microcystic pattern of yolk sac tumor, and the cells of the latter have a more immature primitive appearance with brisker mitotic activity.
Caption: Figure 5: The gross appearance of the cartilage 12 weeks after injection.
Gross appearance of right ovarian papillary serous cystadenoma weighing 2 kg.
Combining the gross appearance and histological results, the hWJMSCs-ECM group yielded higher ICRS gross and histology scores than those in the hWJMSC-Cs-ECM group.
Liu and Guo reviewed the different gross appearance of the lesions and identified single or multiple nodules similar to infarctions [13].
a) Physical parameters: Post-operatively, the animals were checked three times daily for the clinical parameters of temperature, pulse, respiration, vomiting, diarrhea, blood in feces, gross appearance and peritonitis.
The gross appearance of both adult and juvenile GCTs are of a large, tan-yellow tumor with cystic, solid, and hemorrhagic components.
Interstitial pneumonia was diagnosed on the basis of gross appearance, and viral pneumonia or acute bovine pulmonary emphysema and edema, especially that caused by toxic Perilla mint (Peri indicutescens), was suspected.
Early radiograph descriptions, however, have reported on a characteristic concentrically lamellated pattern of splenic calcifications in chronic brucellosis, (3) which unfortunately has not been demonstrated after the advent of CT, (1) probably due to the rarity of splenic brucellomas and the much larger trend for an uncharacteristic gross appearance. Thus, to the best of our knowledge, this is the first report to demonstrate CT and MRI findings of this pathognomonic pattern of calcifications in splenic brucellomas since its earlier radiographic descriptions.
Pathological gross appearance shows a firm circumscribed poorly defined fibro-fatty tissue usually present in the deep dermis and subcutis.
This is an aggressive malignant cancerous tumour and I was concerned that the gross appearance in the abdomen was consistent with this potentially fatal condition.