Answer:
A potentially life-threatening mass of cancerous cells.
Malignant tumors are ambitious. Unlike benign tumors that generally stay put, malignant tumors have two goals in life: to survive and to conquer new territory.
So, if you have a malignant tumor in your colon, it's going to try to work its way through your colon. If successful, it will see where else it can go. This is called metastasizing.
In general, malignant tumors grow faster than benign tumors and are more likely to cause health problems.