Expanded memory, also know as EMS (Expanded Memory Specification) was a method used to increase the 640KB upper limit of MS-DOS to 1MB using a gated memory riser card.
After the advent of extended memory, programs written to use EMS would be redirected by driver emulation to use extended memory instead, which did not rely upon special hardware but just increasing RAM capacities. Extended memory can only be addressed by software running in protected mode as all but the small portion of high memory are unreadable by programs running in standard real mode.
Expanded memory is a distinct technology from the similarly named extended memory, which replaced EMS with greater capacity when it was introduced with the Intel 80286.
Fun fact: The 640KB upper limit of DOS is the alleged source of the often-quoted but steadfastly denied Bill Gates statement: “640KB ought to be enough for anyone.”