https://dev.mysql.com/doc/refman/5.5/en/glossary.html#glos_buffer_pool
buffer pool
The memory area that holds cached InnoDB data for both tables and indexes. For efficiency of high-volume read operations, the buffer pool is divided into pages that can potentially hold multiple rows. For efficiency of cache management, the buffer pool is implemented as a linked list of pages; data that is rarely used is aged out of the cache, using a variation of the LRU algorithm. On systems with large memory, you can improve concurrency by dividing the buffer pool into multiple buffer pool instances.
Several InnoDB status variables, INFORMATION_SCHEMA tables, and performance_schema tables help to monitor the internal workings of the buffer pool. Starting in MySQL 5.6, you can avoid a lengthy warmup period after restarting the server, particularly for instances with large buffer pools, by saving the buffer pool state at server shutdown and restoring the buffer pool to the same state at server startup. See Saving and Restoring the Buffer Pool State.
See Also buffer pool instance, LRU, page, warm up.
Any of the multiple regions into which the buffer pool can be divided, controlled by the innodb_buffer_pool_instances configuration option. The total memory size specified by innodb_buffer_pool_size is divided among all buffer pool instances. Typically, having multiple buffer pool instances is appropriate for systems that allocate multiple gigabytes to the InnoDB buffer pool, with each instance being one gigabyte or larger. On systems loading or looking up large amounts of data in the buffer pool from many concurrent sessions, having multiple buffer pool instances reduces contention for exclusive access to data structures that manage the buffer pool.
See Also buffer pool.