Final answer:
A Type II error occurs when a false null hypothesis is not rejected, denoted by beta (β) in statistics.
Step-by-step explanation:
A Type II error, or a false negative, refers to a situation when the decision is not to reject the null hypothesis when, in fact, the null hypothesis is false. In statistical hypothesis testing, this can be a crucial error as it leads to the incorrect conclusion that there is no effect or difference when one actually exists. The probability of making a Type II error is denoted by the Greek letter beta (β), and efforts are often made in statistical testing to minimize this probability along with the probability of making a Type I error, which happens when the null hypothesis is incorrectly rejected.