B-tree
Balanced search tree designed for block-based storage — each node maps to a disk page and holds hundreds of keys, so a lookup takes only 3–4 disk I/Os even for billion-row tables.
Bayer & McCreight (1972). Default index type in nearly every relational database (Postgres, MySQL/InnoDB, SQL Server, SQLite). The B+-tree variant stores data only in leaves and links leaves so range scans become sequential. Great for OLTP with many point reads/writes.
Loses to LSM-trees on write-heavy workloads (writes amplify via random I/O) — which is why RocksDB, Cassandra and Scylla pick LSM.