Two pointers
Also known as: Two-pointer technique, Twin pointers
The two pointers technique walks an array (or string) with two indices that only move forward (or toward each other), turning a nested $O(n^2)$ scan into $O(n)$ when the structure allows it.
Patterns: opposite ends on a sorted array (pair sum), fast/slow (cycle detection, middle node), and sliding windows (subarray constraints). Often pairs with a prior sort.
Same complexity class as a careful single pass; the point is the invariant that lets you advance a pointer permanently without backtracking.