1. The term "collectively exhaustive" is used in probability and set theory to describe a set of events or subsets that cover the entire sample space or universal set. 2. This means that at least one of the events must occur, or equivalently, the union of all the events equals the entire sample space. 3. For example, if $A_1, A_2, \ldots, A_n$ are events, they are collectively exhaustive if $$\bigcup_{i=1}^n A_i = S,$$ where $S$ is the sample space. 4. This concept ensures that no possible outcome is left out when considering these events together. 5. It is often used alongside the concept of mutually exclusive events, where events cannot happen simultaneously. In summary, "collectively exhaustive" means the events together cover all possible outcomes.