Unsparing, harsh, or strict, as in treatment of others: a severe critic. Marked by or requiring strict adherence to rigorous standards or high principles: a severe code of behavior. Stern or forbidding, as in manner or appearance: spoke in a severe voice. Extremely plain in substance or style: a severe black dress. Causing great discomfort, damage, or distress: a severe pain; a severe storm. Very dangerous or harmful; grave or grievous: severe mental illness. Extremely difficult to perform or endure; trying: a severe test of our loyalty.