Data structure
Organizing data in memory to make certain operations efficient. Picking the right structure is often the difference between O(n) and O(log n).
Fundamentals: array, linked list, stack, queue, hash table, tree (BST, B-tree, trie), heap, graph. Specialized: bloom filter, skip list, fenwick tree, segment tree, suffix array. Robert Tarjan, Don Knuth + The Art of Computer Programming are the reference texts. Modern: persistent/immutable structures (Clojure, Scala), CRDTs for distributed apps.