p Value: The measured probability of a finding occurring, i.e. rejecting the null hypothesis, by chance alone given that the null hypothesis is actually true. By convention, a p value < 0.05 is often considered significant. ("There is less than a 5% probability that the finding [null hypothesis rejected] was due to chance alone.")

From Evidence-Based Medicine Glossary: http://www.musc.edu/muscid/glossary.html