Tumor (American English) or tumour (British English) originally means "swelling", and is sometimes still used with that meaning. Tumor meaning swelling is one of the five classical characteristics of inflammation. However, the term is now primarily used to denote abnormal growth of tissue. This growth can be either (malignant or benign).

Malignant tumors are cancer and invade and destroy neighboring tissues and can become metastatic. Benign tumors do not invade neighboring tissues and do not seed metastases, but may locally grow to great size. Benign tumors usually do not return after surgical removal. Benign tumors are not cancer.

A tumor in epithelium is considered malignant if it penetrates the basal lamina and is considered benign if it does not.

Tumors are caused by mutations in DNA of cells. An accumulation of mutations is needed for a tumor to emerge. Mutations that activate oncogenes or repress tumor supressor genes can eventually lead to tumors. Cells have mechanisms that repair DNA and other mechanisms that cause the cell to destroy itself by commiting apoptosis if DNA damage gets too severe. Mutations that repress the genes for these mechanisms can also eventually lead to cancer. A mutation in one oncogen or one tumor repressor gen is usually not enough for a tumor to occur. A combination of a number of mutations is necessary.

DNA microarrays can be used to determine if oncogens or tumor repressor gens have been mutated. Possibly in the future tumors can be treated better by using DNA microarrays to determine the exact characteristics of the tumor.

As people get older, they accumulate more mutations in their DNA. This means that the prevalence of tumors increases strongly with increasing age. It is also the case that the older a person with a tumor is, the higher the chances are that the tumor is malignant. If a woman of 20 years old has a tumor in her breast it is very likely that the tumor is benign. However, if a woman of 70 has a tumor in her breast it is almost certain that it is malignant.