Fundamentals of Statistics contains material of various lectures and courses of H. Lohninger on statistics, data analysis and chemometrics......click here for more. 
See also: distributions, The Term "significant"  
Level of Significance
When observing a random process, there is always a certain probability that the results are above some threshold. The probability of this threshold being exceeded is called the level of significance α. Statisticians usually say "statistically significant at the 5% level", which means that there is 5% chance that the observed value occurs at random.^{(1)} This term originates from the theory of statistical tests, where the random process is the test statistic. If the test statistic exceeds the limit z_{x} (the critical threshold), the probability of making a type I error is α.
The shaded area of the distribution density above specifies the probability that the outcome z of an experiment exceeds the threshold value z_{x}. Please note that in some cases, the interesting question may implicitly refer to two thresholds, as shown below:
In this case, the level of significance is the sum of the areas below z_{x} and above +z_{x}.


Last Update: 20121008