The easy definition is:
Malignant = Cancerous
Benign = Non-Cancerous
What does malignant mean?
I mean, I know it means cancer, but what else?
Here is what wikipedia says: Malignancy (from Latin male "badly" + -gnus "born") is the tendency of a medical condition, especially tumors, to become progressively worse and to potentially result in death. Malignancy in cancers is characterized by anaplasia, invasiveness, and metastasis.
Actually, long before the medical community started using it, malignant meant evil, malevolent, malicious, deadly. As a medical term it means capable of invading into adjacent tissues, and may be capable of spreading to distant tissues.This medical definition applies to cancer as well as other medical conditions such as hypertension or hyperthermia where the condition can spread fro a primary area in the body to damage other areas of the body.