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

Synonyms: These adjectives mean unsparing and exacting with respect to discipline or control. Severe implies adherence to rigorous standards or high principles and often suggests harshness: "Praise or blame has but a momentary effect on the man whose love of beauty in the abstract makes him a severe critic on his own works" (John Keats).
Stern suggests unyielding disposition, uncompromising resolution, or forbidding appearance or nature: "a man fatally stern and implacable" (George Meredith).
Austere connotes aloofness or lack of feeling or sympathy, and often rigid morality: Austere officers demand meticulous conformity with military regulations.
Ascetic suggests self-discipline and often renunciation of worldly pleasures for spiritual improvement: "Be systematically ascetic ... do ... something for no other reason than that you would rather not do it" (William James).
Strict means requiring or showing stringent observance of obligations, rules, or standards: "He could not be severe nor even passably strict" (W.H. Hudson).