The term is typically seen when referring to security products on IBM z/OS platforms. An example is the RACF security package. [1]
External Security Managers (ESM) is best known on IBM Mainframe
On z/OS there are three primary External Security Managers (ESM)
In 1987, acquisitions made Computer Associates the largest independent vendor of mainframe infrastructure software and dominant vendor of OS/MVS security software with CA-Top Secret (#2 market share) and CA-ACF2 (#1 market share). IBM's RACF product held the #3 market share position. I really have no idea where market share of these products are today.
External Security Managers (ESM) is an Authentication Agent for z/OS platforms and can be utilized to provide Single Sign-On for applicaitons on the platform.
There is a DirXML ZOS BIDIRECTIONAL DRIVER that will work with most of the External Security Managers (ESM) on z/OS platforms.