A Log-Structured Merge-tree (LSM tree) is a data structure designed for efficient write operations in databases. It organizes data in a way that allows for fast inserts and updates by writing changes to a memory-resident structure before merging them into a disk-based structure. This approach minimizes the need for immediate disk writes, improving performance for workloads with high write volumes. The LSM tree consists of multiple levels, where data is periodically merged and sorted. This merging process helps maintain read efficiency by ensuring that data is organized and accessible. Overall, LSM trees are commonly used in modern NoSQL databases to optimize performance and storage.