In MySQL, an index is a data structure that improves the speed of operations in a table. Indexes are used to retrieve data from the database more quickly than otherwise, and they are implemented using additional data structures that are stored alongside the table data. Without an index, MySQL must scan the whole table to locate the relevant rows, which can be very slow for large tables.

MySQL uses indexes for several operations, including finding rows matching a WHERE clause quickly, eliminating rows from consideration, retrieving rows from other tables when performing joins, and optimizing MIN() and MAX() functions for specific indexed column key_col. Indexes can be created using one or more columns, providing the basis for both rapid random lookups and efficient ordering of access to records.

Creating an index in MySQL involves using the CREATE INDEX statement, which creates an index on a table. Duplicate values are allowed, and the users cannot see the indexes, they are just used to speed up searches/queries. Updating a table with indexes takes more time than updating a table without, so it is recommended to only create indexes on columns that will be frequently searched against.

In summary, an index in MySQL is a data structure that improves the speed of operations in a table, and it is used to retrieve data from the database more quickly than otherwise. MySQL uses indexes for several operations, and they can be created using one or more columns.