"O(n log n)" is a notation used in computer science to describe the time complexity of certain algorithms. It indicates that the time it takes to complete a task grows in proportion to the size of the input data, multiplied by the logarithm of that size. This complexity is often seen in efficient sorting algorithms, such as Merge Sort and Quick Sort, which divide the data into smaller parts and sort them recursively. In practical terms, if you have a list of items to sort, an algorithm with O(n log n) complexity will generally perform better than one with O(n²) complexity as the list size increases. This efficiency makes O(n log n) algorithms suitable for handling large datasets, ensuring that operations remain manageable even as the amount of data grows.