A tumor or tumour is the name for a neoplasm or a solid lesion formed by an abnormal growth of cells (termed neoplastic) which looks like a swelling. Tumor is not synonymous with cancer. A tumor can be benign, pre-malignant or malignant, whereas cancer is by definition malignant.
The term tumour/tumor is derived from the Latin word for "swelling" tumor and has come to the English language via the Old French tumour (contemporary French: tumeur). In the Commonwealth the spelling "tumour" is commonly used, whereas in the U.S. it is usually spelled "tumor".
In its medical sense it originally meant an abnormal swelling of the flesh. Celsus (ca 30 BC–38 AD) described four cardinal signs of acute inflammation as tumor, dolor, calor and rubor (swelling, pain, increased heat and redness).
But 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. Swelling is most often caused by inflammation caused by trauma, infection, etc.
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.
The nature of the tumor is determined by a pathologist after examination of the tumor tissues from a biopsy or a surgical excision specimen and is then qualified as benign, pre-malignant or malignant.
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