The notation "n \choose k" represents a binomial coefficient, which counts the number of ways to choose k items from a set of n distinct items without regard to the order of selection. It is calculated using the formula: n \choose k = \fracn!k!(n-k)!, where "!" denotes factorial, the product of all positive integers up to that number. This concept is widely used in combinatorics, probability, and statistics. For example, it helps determine the number of possible combinations in a lottery or the ways to form teams from a group. Understanding "n \choose k" is essential for solving various problems in these fields.