In common usage, the word "probability" is used to mean the chance that a particular event (or set of events) will occur expressed on a linear scale from 0 (impossibility) to 1 (certainty), also expressed as a percentage between 0 and 100%. In the fuzzy domain the scale ranges from -1 (impossible) through 0 (unknown) to 1 (certainty).
From 
Mathworld:Probability.