Stable sort:
A sorting algorithm is stable if, when two elements have the same key (the value being compared), their relative order from the original input is preserved in the sorted output.
Examples: Merge Sort, Insertion Sort

Unstable sort example:
Suppose you’re sorting an array of tuples by the first element only:
[(1, a), (4, a), (3, x), (4, b)]
Sorted by first element → (keys: 1, 4, 3, 4):
[(1, a), (3, x), (4, b), (4, a)] # 4a and 4b order swapped
Here, (4, a) originally came before (4, b), but after sorting, it’s after.
So the algorithm is unstable.