rig·or 
(rĭgər)
(rĭgər)n.
1.
a. Strictness or severity, as in action or judgment: "The desert fostered a closed world of faith and rigor and harsh judgment: almost every decision here could have lethal consequences" (Jeffrey Tayler).
b. A harsh or trying circumstance; a hardship or difficulty: the rigors of working in a coal mine. See Synonyms at difficulty.
c. Archaic A harsh or severe act.
2.
a. Strictness in adhering to standards or a method; exactitude: "To study the brain with scientific rigor, behaviorists logically restricted their experiments to ones in which the brain was the source of measurable effects" (Robert Pollack).
b. A standard or exacting requirement, as of a field of study: the intellectual rigors of advanced mathematics.
3. Medicine Shivering or trembling, as caused by a chill.
4. Physiology A state of rigidity in living tissues or organs that prevents response to stimuli.
5. Obsolete Stiffness or rigidity.
[Middle English rigour, from Old French, from Latin rigor, from rigēre, to be stiff; see reig- in the Appendix of Indo-European roots.]