Final answer:
A malignant tumor is a cancerous mass of cells that grow uncontrollably, invade neighboring tissues, and can spread to other parts of the body.
Step-by-step explanation:
A malignant tumor is a cancerous mass of cells that grow uncontrollably and can invade and damage neighboring tissues. Unlike benign tumors, malignant tumors can spread to other parts of the body through the bloodstream, leading to the formation of new tumors. They are the most deadly form of cancer.