I searched Wiki but there seems be no definition for this abbreviative word.

And it is explained like this on website of http://www.epssinfosite.com/:

EPSS is defined as an integrated electronic environment that is available to and easily accessible by each employee and is structured to provide immediate, individualized on-line access to the full range of information, software, guidance, advice and assistance, data, images, tools, and assessment and monitoring systems to permit job performance with minimal support and intervention by others.

In the ground-breaking book Electronic Performance Support Systems published in 1991, Gloria Gery, defined EPSS as:

“an integrated electronic environment that is available to and easily accessible by each employee and is structured to provide immediate, individualized on-line access to the full range of information, software, guidance, advice and assistance, data, images, tools, and assessment and monitoring systems to permit job performance with minimal support and intervention by others.”

I think this link will be a helpful one from San Diego State University to understand what is EPSS: