P-value
Short definition
The p-value is the probability of obtaining results at least as extreme as those observed if there were in fact no effect or difference (the null hypothesis).
In plain language
By convention, a p-value below 0.05 is often called «statistically significant». But the p-value does not tell you the probability that the intervention works, nor the size of the effect, nor whether it is clinically relevant. In 2016 the American Statistical Association warned that conclusions should not be based solely on whether a p-value passes a threshold; it should be read together with the effect size and the confidence interval.