A condition which, if true, guarantees that a result is also true. (However, the result may also be true if the condition
is not met.) If a condition is both necessary
and sufficient, then the result is said to be true iff ("if
and only if") the condition holds.
For example, the condition that a decimal number end in the digit 2 is a sufficient
but not necessary condition that be even.
Jeffreys, H. and Jeffreys, B. S. "Necessary: Sufficient." §1.036 in Methods
of Mathematical Physics, 3rd ed. Cambridge, England: Cambridge University
Press, pp. 10-11, 1988.
end in the digit 2 is a sufficient
but not necessary condition that 
be even.
