A malignant tumor is different from a benign tumor in that a malignant tumor is

cancerous

Explanation:

The malignant tumor has the ability of metastasis which in turn makes it cancerous. The cells of the malignant tumors have the ability to migrate from one body part to others. Their ability of movement between various body parts enables them to invade various tissues and form multiple tumors. This finally leads to cancer.

The cells of a benign tumor can not invade other tissues or other body parts, that is, they are localized and non-cancerous.

The answer is ... a cancerous tumor. Benign means that it is cancer-free.