The uncontrolled growth of cells is called tumour or neoplasm. Tumours are considered to be either malignant or benign. Those tumours that are stagnant at one part of the body and do not spread are called benign tumours. But certain tumours invade and damage the surrounding tissues. Such tumours are known as malignant tumours. Only malignant tumours are properly designated as cancerous. Normal cells show a property called contact inhibition. Due to this property, when they are in contact with other cells the cell proliferation ceases. Cancer cells do not show the property of contact inhibition. A phenomenon in which the cancerous cells spread to distant sites in the body is called metastasis.