A mathematical notation that describes the upper bound of an algorithm's time or space complexity as input size grows. Common complexities from fastest to slowest: O(1), O(log n), O(n), O(n log n), O(n^2), O(2^n).

A data structure consisting of nodes (vertices) connected by edges. Graphs can be directed or undirected, weighted or unweighted. Used to model networks, social connections, maps, and dependencies.

A technique where a function calls itself to solve smaller instances of the same problem. Every recursive function needs a base case (stopping condition) and a recursive case. Common in tree traversals, divide-and-conquer algorithms, and mathematical computations.