In hypothesis testing, the probability of a Type II error, i.e. the probability of concluding incorrectly that a null hypothesis is true.
Note: For example, β could be the probability of concluding that an intervention is not effective when it has a true effect. (1-β) is the statistical power of a test allowing for rejection of a null hypothesis that is truly false (e.g. detecting the effect of an intervention that truly exists).
(Related concepts: hypothesis testing, statistical power)