[too-mer, tyoo-]
tumor: see neoplasm.
or neoplasm

Mass of abnormal tissue that arises from normal cells, has no useful function, and tends to grow. Cell abnormalities may include increased size or number or loss of characteristics that differentiate their tissue of origin. Cells in malignant tumours (see cancer) have a distorted size, shape, and/or structure. Less differentiated cells tend to grow faster. Malignant tumours invade tissues locally and spread (metastasize) in blood or lymph: the stronger the tendency to metastasize, the more malignant the tumour. Tumours may not cause pain until they press on or invade nerves. Both benign and malignant tumours can press on nearby structures, block vessels, or produce excess hormones, all of which can cause death. Benign tumours remain as a solid mass that can be removed by surgery if accessible; they can consist of various tissues and may become malignant; malignant tumours, though they may remain quiescent for a time, never become benign.

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A tumor or tumour is the name for a swelling or lesion formed by an abnormal growth of cells (termed neoplastic). Tumor is not synonymous with cancer. A tumor can be benign, pre-malignant or malignant, whereas cancer is by definition malignant.


The term tumor is derived, via the Old French tumour, from the Latin tumor "swelling". It originally meant an abnormal swelling of the flesh. In contemporary English, tumor is synonymous with solid neoplasm , all other forms of swelling being called swelling. This usage is common also in medical literature, where the nouns tumefaction and tumescence, derived from the adjective tumefied, are the current medical terms for non-neoplastic swelling.


A neoplasm is an abnormal proliferation of tissues, usually caused by genetic mutations. Most neoplasms cause a tumor, with a few exceptions like leukemia or carcinoma in situ.

Tumors may be benign, pre-malignant or malignant (cancer). The nature of the tumor is determined by a pathologist after examination of the tumor tissues from a biopsy or a surgical excision specimen.

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