What does "in-place" mean regarding sorting algorithms?

The term "in-place" in the context of sorting algorithms refers specifically to the sorting process occurring within the same array that is being sorted. This means that the algorithm does not require any additional space proportional to the size of the input for the purpose of temporarily storing elements during the sort. Instead, it rearranges the elements of the original array itself to achieve the sorted order.

This characteristic is significant as it allows for more memory-efficient sorting—ideal when working with large datasets where memory usage is a critical consideration. In-place algorithms typically maintain a constant amount of extra space, often O(1), for variables needed during the sorting process, which helps in large data manipulation scenarios.

In contrast, using additional arrays or external files for sorting would require extra space and go against the definition of in-place sorting. Likewise, utilizing multiple threads does not pertain to the characteristic of in-place sorting; it focuses on the method of execution rather than the memory usage aspect. Thus, sorting done within the same array is the defining feature of an in-place sorting algorithm.