severe

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Unsparing, harsh, or strict, as in treatment of others.
• adjective Marked by or requiring strict adherence to rigorous standards or high principles.
• adjective Stern or forbidding, as in manner or appearance.
• adjective Extremely plain in substance or style.
• adjective Causing great discomfort, damage, or distress.
• adjective Very dangerous or harmful; grave or grievous.
• adjective Extremely difficult to perform or endure; trying.

from The Century Dictionary.

• Serious or earnest in feeling, manner, or appearance; without levity; sedate; grave; austere; not light, lively, or cheerful.
• Very strict in judgment, discipline, or ac tion; not mild or indulgent; rigorous; harsh; rigid; merciless: as, severe criticism; severe punishment.
• Strictly regulated by rule or principle; exactly conforming to a standard; rigidly methodical; hence, in lit., art, etc., avoiding, or not exhibiting or permitting, unnecessary or florid ornament, amplification, or the like; restrained; not luxuriant; always keeping measure; pure in line and form; chaste in conception; subordinated to a high ideal: as, a severe style of writing; the severest style of Greek architecture; the severe school of German music.
• Sharp; afflictive; distressing; violent; extreme: as, severe pain, anguish, or torture; severe cold; a severe winter.
• Difficult to be endured; trying; critical; rigorous: as, a severe test; a severe examination.
• Synonyms and Harsh, Strict, etc. (see austere), unrelenting.
• Exact, accurate, unadorned, chaste.
• Cutting, keen, biting.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Serious in feeling or manner; sedate; grave; austere; not light, lively, or cheerful.
• adjective Very strict in judgment, discipline, or government; harsh; not mild or indulgent; rigorous.
• adjective Rigidly methodical, or adherent to rule or principle; exactly conformed to a standard; not allowing or employing unneccessary ornament, amplification, etc.; strict; -- said of style, argument, etc.
• adjective Sharp; afflictive; distressing; violent; extreme.
• adjective Difficult to be endured; exact; critical; rigorous.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Very bad or intense.
• adjective Strict or harsh.
• adjective Sober, plain in appearance, austere.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective unsparing and uncompromising in discipline or judgment
• adjective severely simple
• adjective intensely or extremely bad or unpleasant in degree or quality
• adjective causing fear or anxiety by threatening great harm
• adjective very strong or vigorous
• adjective very bad in degree or extent