When we approximate some other distribution using the Normal distribution. For example, if we have tossed 100 coins and counted the number of heads, this is strictly a Binomial distribution; however, we might approximate this by a Normal distribution with mean of 50 and standard deviation of 0.05.
Also used in hcistats2e: Chap. 4: page 50
Used in glossary entries: Binomial distribution, mean (μ), Normal distribution, standard deviation (s.d., σ)