rigorous

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

• adjective Characterized by or adhering to strict standards or methods; exacting and thorough.
• adjective Harsh, severe, or demanding.
• adjective Very unpleasant or inclement.
• adjective Adhering strictly to a belief or ideology; uncompromising or inflexible.

from The Century Dictionary.

• Acting with rigor; strict in performance or requirement.
• Marked by inflexibility or severity; stringent; exacting; hence, unmitigated; merciless.
• Exact; strict; precise; scrupulously accurate: as, a rigorous definition or demonstration.
• Hard; inclement; bitter; severe: as, a rigorous winter.
• Synonyms and Severe, Rigid, etc. (see austere), inflexible, unbending, unyielding.

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

• adjective Manifesting, exercising, or favoring rigor; allowing no abatement or mitigation; scrupulously accurate; exact; strict; severe; relentless
• adjective Severe; intense; inclement.
• adjective obsolete Violent.
• adjective (Mathematics, Logic) Adhering scrupulously and exactly to accepted principles; hence, logically valid.

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

• adjective Manifesting, exercising, or favoring rigour; allowing no abatement or mitigation; scrupulously accurate; exact; strict; severe; relentless; as, a rigorous officer of justice; a rigorous execution of law; a rigorous definition or demonstration.
• adjective Severe; intense; inclement; as, a rigorous winter.
• adjective Violent.

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

• adjective demanding strict attention to rules and procedures
• adjective rigidly accurate; allowing no deviation from a standard