Final answer:
The ISTQB refers to the phenomenon where a majority of defects are found in a small number of modules as the Pareto Principle, commonly known as the 80/20 rule.
Step-by-step explanation:
According to the International Software Testing Qualifications Board (ISTQB), the phenomenon where a small number of modules contain most of the defects discovered during pre-release testing or show the most operational failures is called the Pareto Principle. This principle, also known as the 80/20 rule, is a common pattern in software development and testing where roughly 80% of problems are found in 20% of the components. The idea is that most defects are not evenly distributed, but tend to be focused in a small number of areas, which makes it crucial for testers to identify these error-prone modules to efficiently manage testing and debugging.