These two opposing terms are used to describe key characteristics of a tumor. [Benign] derives from the Latin term [benignus], meaning "good" and "gentle". The medical application of the word denotes a condition (or tumor) that is not bad (malignant), and that it is favorable for treatment and recovery.
The term [malignant] derives from the Latin word [malus], meaning "ill-disposed, malicious, or bad". The medical application is to a condition or tumor that is unfavorable to treatment and recovery. Over time the term malignant has become synonymous with "cancer".