In medicine, benign means a growth is not cancerous and is unlikely to spread, while malignant means it is likely to spread and become dangerous. Outside the doctor's office, the same contrast holds: benign suggests something kind and harmless, malignant something hostile and harmful.
The core difference
Benign means harmless or non-cancerous; malignant means harmful or cancerous and likely to spread.
- benign — not cancerous and unlikely to spread, or kind and well-meaning: The doctors confirmed the lump was benign and needed no further treatment.
- malignant — likely to spread and become life-threatening, or harmful and hostile: The biopsy confirmed the growth was malignant.
How to tell them apart
Both words carry a medical sense and a character sense, and the two line up neatly. A benign tumour, like a benign smile, is well-meaning and unlikely to cause serious harm, though a benign tumour can still cause problems if it grows large enough. A malignant tumour, like a malignant rumour, is aggressive by nature and tends to spread.
The safest way to remember which is which in a medical context is that malignant is the one to worry about: it is the word doctors use when cells are likely to spread beyond where they started.
