The term "n-k" typically refers to a mathematical expression where "n" represents a total number of items, and "k" signifies the number of items being selected or removed. This concept is often used in combinatorics, a branch of mathematics that deals with counting, arrangement, and combination of objects. The expression helps in calculating the number of ways to choose "k" items from a set of "n" items. In practical applications, "n-k" can be seen in various fields such as statistics, computer science, and probability theory. For example, in statistics, it can help determine sample sizes, while in computer science, it may be used in algorithms that require selection or filtering of data. Understanding "n-k" is essential for solving problems related to combinations and permutations.