An indexed file is a computer file with an index that allows easy random access to any record given its file key.
The key must be such that it uniquely identifies a record. If more than one index is present the other ones are called alternate indexes. The indexes are created with the file and maintained by the system.
Support for indexed files is built into COBOL and PL/I. Other languages with more limited I/O facilities such as C support indexed files through add-on packages in a runtime library such as C-ISAM.
The COBOL language supports indexed files with the following command in the FILE CONTROL section
ORGANIZATION IS INDEXED
In recent systems relational databases are often used in place of indexed files.
References
Indexed file Wikipedia(Text) CC BY-SA