Algorithm

A finite, unambiguous procedure for turning an input into an output — sorting a list, finding a path, matching a pattern — judged by whether it is correct and by how its cost grows as the input grows

Big-O notation describes that growth, not that speed: O(n²) beats O(n log n) at small sizes often enough that the constant matters more than the exponent in real code. The exponent starts to matter at the size where an input doubling hurts, and the practical skill is recognizing which loop is the one that will meet a hundred thousand rows.

In common usage, the ranking or recommendation system a platform applies to a feed — the algorithm

Worth separating because the word does different work in the two sentences. A sorting algorithm is a procedure you can read; the algorithm is a trained model whose behavior nobody can fully state, including the people who deployed it. When someone says a post was hidden by the algorithm, the first sense is not what they mean.